Leadless cardiac stimulation systems

ABSTRACT

Various configurations of systems that employ leadless electrodes to provide pacing therapy are provided. In one example, a system that provides multiple sites for pacing of myocardium of a heart includes wireless pacing electrode assemblies that are implantable at sites proximate the myocardium using a percutaneous, transluminal, catheter delivery system. Also disclosed are various configurations of such systems, wireless electrode assemblies, and delivery catheters for delivering and implanting the electrode assemblies.

CLAIM OF PRIORITY

This application is a continuation of U.S. patent application Ser. No.13/717,027, filed on Dec. 17, 2012, pending, which is a continuation ofU.S. patent application Ser. No. 11/745,105, filed on May 7, 2007, nowissued as U.S. Pat. No. 8,340,780, which is a divisional of U.S. patentapplication Ser. No. 11/075,376 filed on Mar. 7, 2005, now issued asU.S. Pat. No. 7,647,109, which is a continuation-in-part of U.S. patentapplication Ser. No. 10/971,550, filed on Oct. 20, 2004, now issued asU.S. Pat. No. 7,532,933, the benefit of priority of each of which ishereby presently claimed, and the entire contents of each of which ishereby incorporated by reference.

TECHNICAL FIELD

This document relates to systems that electrically stimulate cardiac orother tissue and that do so without using leads that extend into theheart or other surrounding tissue or organs, along with systems andmethods for introducing such stimulators.

BACKGROUND

Pacemakers provide electrical stimulus to heart tissue to cause theheart to contract and hence pump blood. Conventionally, pacemakersinclude a pulse generator that is implanted, typically in a patient'spectoral region just under the skin. One or more leads extend from thepulse generator and into chambers of the heart, most commonly into theright ventricle and the right atrium, although sometimes also into avein over the left chambers of the heart. An electrode is at a far endof a lead and provides the electrical contact to the heart tissue fordelivery of the electrical pulses generated by the pulse generator anddelivered to the electrode through the lead.

The conventional use of leads that extend from the pulse generator andinto the heart chambers has various drawbacks. For example, leads haveat their far ends a mechanism, such as tines or a “j-hook,” that causesthe lead to be secured to a tissue region where a physician positionsthe lead. Over time, the heart tissue becomes intertwined with the leadto keep the lead in place. Although this is advantageous in that itensures the tissue region selected by the physician continues to be theregion that is paced even after the patient has left the hospital, it isalso disadvantageous in the event of a lead failure or in the event itis later found that it would be more desirable to pace a differentlocation than the tissue region initially selected. Failed leads cannotalways be left in the patient's body, due to any potential adversereaction the leads may have on heart function, including infection,thrombosis, valve dysfunction, etc. Therefore, difficult lead removalprocedures sometimes must be employed.

The conventional use of leads also limits the number of sites of hearttissue at which electrical energy may be delivered. The reason the useof leads is limiting is that leads most commonly are positioned withincardiac veins. As shown in FIG. 17, up to three leads 2, 3 and 4 areimplanted in conventional pacing systems that perform multiple-sitepacing of the heart 1, with the leads exiting the right atrium 5 via thesuperior vena cava 6. Multiple leads may block a clinically significantfraction of the cross section of the vena cava and branching veinsleading to the pacemaker implant.

No commercial pacing lead has been indicated for use in the chambers ofthe left side of the heart. This is because the high pumping pressure onthe left side of the heart may eject a thrombus or clot that forms on alead or electrode into distal arteries feeding critical tissues andcausing stroke or other embolic injury. Thus, conventional systems, asshown in FIG. 17, designed to pace the left side of the heart thread apacing lead 2 through the coronary sinus ostium 7, located in the rightatrium 5, and through the coronary venous system 8 to a location 9 in avein over the site to be paced on the left side. While a single lead mayocclude a vein over the left heart locally, this is overcome by the factthat other veins may compensate for the occlusion and deliver more bloodto the heart. Nevertheless, multiple leads positioned in veins wouldcause significant occlusion, particularly in veins such as the coronarysinus that would require multiple side-by-side leads.

There are several heart conditions that may benefit from pacing atmultiple sites of heart tissue. One such condition is congestive heartfailure (CHF). It has been found that CHF patients have benefited frombi-ventricular pacing, that is, pacing of both the left ventricle andthe right ventricle in a timed relationship. Such therapy has beenreferred to as “resynchronization therapy.” It is believed that manymore patients could benefit if multiple sites in the left and rightventricles could be synchronously paced. In addition, pacing at multiplesites may be beneficial where heart tissue through which electricalenergy must propagate is scarred or dysfunctional, which condition haltsor alters the propagation of an electrical signal through that hearttissue. In these cases multiple-site pacing may be useful to restart thepropagation of the electrical signal immediately downstream of the deador sick tissue area. Synchronized pacing at multiple sites on the heartmay inhibit the onset of fibrillation resulting from slow or aberrantconduction, thus reducing the need for implanted or external cardiacdefibrillators. Arrhythmias may result from slow conduction orenlargement of the heart chamber. In these diseases, a depolarizationwave that has taken a long and/or slow path around a heart chamber mayreturn to its starting point after that tissue has had time tore-polarize. In this way, a never ending “race-track” or “circus” wavemay exist in one or more chambers that is not synchronized with normalsinus rhythm. Atrial fibrillation, a common and life threateningcondition, may often be associated with such conduction abnormalities.Pacing at a sufficient number of sites in one or more heart chambers,for example in the atria, may force all tissue to depolarize in asynchronous manner to prevent the race-track and circus rhythms thatlead to fibrillation.

Systems using wireless electrodes that are attached to the epicardialsurface of the heart to stimulate heart tissue have been suggested as away of overcoming the limitations that leads pose. In the suggestedsystem, wireless electrodes receive energy for generating a pacingelectrical pulse via inductive coupling of a coil in the electrode to aradio frequency (RF) antenna attached to a central pacing controller,which may also be implanted. The wireless electrodes are screwed intothe outside surface of the heart wall.

SUMMARY

The invention is directed to various configurations of systems thatemploy leadless electrodes to provide pacing therapy and that arecommercially practicable. One of the findings of the inventors is that asignificant issue to be considered in achieving a commerciallypracticable system is the overall energy efficiency of the implantedsystem. For example, the energy transfer efficiency of two inductivelycoupled coils decreases dramatically as the distance between the coilsincreases. Thus, for example, a transmitter coil implanted in the usualupper pectoral region may only be able to couple negligible energy to asmall seed electrode coil located within the heart.

One aspect of the invention may include a catheter delivery system forimplantation of at least a portion of a wireless electrode assemblythrough endocardium tissue and into myocardium tissue. The catheterdelivery system may include a first elongate member having a proximalend and a distal end and defining a lumen passing therethrough. Thesystem may also include a second elongate member having a proximal endand a distal end. The system may further include a wireless electrodeassembly attachable to the distal end of the second elongate member.When the electrode assembly is attached to the second elongate member,the second elongate member may be passable through the lumen to deliverat least a portion of the electrode assembly through the endocardium andinto the myocardium.

In some embodiments, the electrode assembly may include an attachmentmechanism that has at least one fastener to penetrate through theendocardium and into the myocardium. The attachment mechanism may beoperable to secure at least a portion of the electrode assembly to themyocardium. In some instances, the attachment mechanism may include atleast one helical tine and at least one curled tine. For example, theattachment mechanism may include a distally extending helical tine topenetrate through the endocardium and into the myocardium and aplurality of radially extending curled tines. In other instances, thefastener of the attachment mechanism may include a tine, screw, barb, orhook.

In further embodiments, the second elongate member may have a detachmentmechanism at the distal end to release the electrode assembly from thesecond elongate member after delivery of the electrode assembly to themyocardium. In some instances, the detachment mechanism may include athreaded member that releasably engages a portion of the electrodeassembly. In other instances, the detachment mechanism may include anadjustable locking member that releasably engages a portion of theelectrode assembly.

In certain embodiments, the first elongate member includes a steeringmechanism to direct the distal end of the first elongate member to aselected site proximate to the endocardium. The first elongate membermay include an electrode at its distal end for sensing a localelectrocardiogram at the selected site proximate to the endocardium.

In some embodiments, the system also includes an access catheter havinga proximal end and a distal end and having a lumen passing therethrough,The first elongate member may be a delivery catheter that is passablethrough the lumen of the access catheter. An image device may bedisposed near the distal end of the access catheter. The image devicemay include an ultrasonic device to provide visualization of a selectedsite distal of the access catheter.

In another aspect, an implantable wireless electrode assembly mayinclude a first electrode to discharge a pacing electrical pulse. Theassembly may also include an attachment mechanism having at least onefastener to penetrate through endocardium tissue and into myocardiumtissue. At least a portion of the attachment mechanism may be disposedproximate to the electrode such that, when the fastener penetratesthrough the endocardium and into the myocardium, the electrode ispositioned proximate to the myocardium.

In some embodiments, the wireless electrode assembly also includes asecond electrode. The second electrode may be spaced apart from thefirst electrode such that, when the fastener penetrates throughendocardium and into the myocardium, the first electrode is positionedproximate to the myocardium while the second electrode is exposed toblood in an internal heart chamber.

In further embodiments, the wireless electrode assembly may also includean induction device to receive electromagnetic energy from an externalsource. The first electrode may be electrically connected to a circuitsuch that the pacing electrical pulse is generated from at least aportion of the electromagnetic energy received by the induction device.The circuit may include an energy storage element to store theelectromagnetic energy received by the induction device. The energystorage element may be operable to periodically discharge electricalenergy to the electrode.

In certain embodiments, the wireless electrode assembly may include ainduction device comprising a coil that is inductively coupled to theexternal source.

In some embodiments, the wireless electrode assembly may include anattachment mechanism that comprises at least one helical tine and atleast one curled tine. The attachment mechanism may include a distallyextending helical tine to penetrate through the endocardium and into themyocardium and may include a plurality of radially extending tines thatare adapted to a curl into the endocardium or myocardium after thehelical tine penetrates into the myocardium.

In other embodiments, the wireless electrode assembly may includeattachment mechanism that comprises a tine, screw, barb, or hook.

In further embodiments, the wireless electrode assembly also includes adetachment mechanism spaced apart from the fastener of the attachmentmechanism. The detachment mechanism may include a threaded member andmay be operable to release the wireless electrode assembly from adelivery system after the fastener penetrates through endocardium andinto the myocardium.

Yet another aspect may include a method of delivering a wirelesselectrode assembly into an internal heart chamber and proximate themyocardium. The method may include directing a distal end of a firstelongate member into an internal heart chamber. The first elongatemember may have the distal end, a proximal end, and a lumen passingtherethrough. The method may also include directing a wireless electrodeassembly through the lumen of the first elongate member toward thedistal end of the first elongate member. The method may further includepenetrating at least a portion of the wireless electrode assemblythrough endocardium tissue and into the myocardium.

In some embodiments, the method may employ a wireless electrode assemblythat is attached to a distal end of a second elongate member. The secondelongate member may be passable through the lumen of the first elongatemember. In such cases, the method may also include operating adetachment mechanism to release the wireless electrode assembly from thefirst elongate member. Furthermore, the method may also includewithdrawing the second elongate member and the first elongate memberaway from endocardium.

In certain embodiments, the method may also include measuring a localelectrocardiogram with a sensor at the distal end of the first elongatemember after at least a portion of the wireless electrode assemblypenetrates the endocardium. In such cases, the method may also includedeploying one or more adjustable tines of the wireless electrodeassembly after measuring the local electrocardiogram. In certaincircumstances, the method may include withdrawing the wireless electrodeassembly from the myocardium after measuring the local electrocardiogramand penetrating at least a portion of the wireless electrode assemblythrough a different portion the endocardium and into a different portionof the myocardium.

In some embodiments, the operation of penetrating at least a portion ofthe wireless electrode assembly through endocardium tissue includescausing an attachment mechanism of the electrode assembly to penetratethrough the endocardium.

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of the invention will be apparent from thedescription and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a conceptual diagram of a leadless cardiac stimulation system(with leadless, or wireless, electrode assemblies shown implanted in aheart) and of an external programmer.

FIGS. 2A and 2B are exemplary systems of the type shown in FIG. 1, andshown implanted in a body.

FIG. 3 is a block diagram of an exemplary embodiment of a combinedcontroller/transmitter device and associated antenna that may be used aspart of the FIG. 2A or 2B system,

FIG. 4 is a schematic diagram of a portion of the circuitry included ina wireless electrode assembly as is shown in FIGS. 1 and 2A-B.

FIG. 5 is a flow chart of a method of providing stimulation pulses in apacing cycle in a system such as shown in FIGS. 1 and 2A-B.

FIG. 6 is a diagram of the system shown in FIG. 2A and of an examplewireless electrode assembly delivery catheter.

FIG. 7 is a side-view diagram of the delivery catheter shown in FIG. 6,with portions removed to show a wireless electrode assembly andadditional assemblies inside the catheter.

FIG. 8 is a diagram similar to FIG. 7, with a distal end of the deliverycatheter pressed against a myocardial wall.

FIG. 9 is a diagram illustrating the delivery of a wireless electrodeassembly from the delivery catheter and into the myocardial wall.

FIG. 10 is a flow chart of a method for delivering and implantingwireless electrode assemblies.

FIGS. 11A-D are diagrams of alternative embodiments of wirelesselectrode assemblies and associated delivery catheters, with thewireless electrode assemblies shown being implanted within a myocardialwall.

FIGS. 11E-W are diagrams of alternative embodiments of wirelesselectrode assemblies and associated delivery catheters.

FIG. 12 is a diagram of a wireless electrode assembly and associateddelivery catheter, with the wireless electrode assembly shown implantedwithin a myocardial wall in a position such that its longitudinal axisis parallel with the myocardial wall.

FIG. 13 is a diagram of a wireless electrode assembly and an anotherembodiment of an associated delivery catheter.

FIGS. 14A and 14B are diagrams of an alternative embodiment of awireless electrode assembly and associated delivery catheter, with thewireless electrode assembly being shown being implanted within amyocardial wall.

FIG. 15 is a diagram of an alternative embodiment of a coil for awireless electrode assembly in which three orthogonal coils are wound ona single substrate.

FIG. 16 is a part schematic and part block diagram of a circuit that maybe included within embodiments of wireless electrode assemblies toenable them to receive and to transmit information.

FIG. 17 is an example of a prior art, three-lead pacing system, showingone lead placed in a vein over the left ventricle.

FIGS. 18A to 18C show views of a wireless electrode assembly and awireless electrode assembly attached to a tissue equivalent circuit.

FIG. 19 is a graph of voltage, both computed and measured, induced in awireless electrode assembly versus time.

FIG. 20 is a graph of voltage induced in a particular wireless electrodeassembly versus time, with and without a tissue equivalent circuitattached across the electrodes.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

This document describes various configurations of systems that employleadless electrodes to provide pacing therapy or other tissue excitationand that are commercially practicable. One of the findings of theinventors is that a significant issue to be considered in achieving acommercially practicable system is the overall energy efficiency of theimplanted system. For example, the energy transfer efficiency of twoinductively coupled coils decreases dramatically as the distance betweenthe coils increases. Thus, for example, a transmitter coil implanted inthe usual upper pectoral region may only be able to couple negligibleenergy to a small seed electrode coil located within the heart.

FIG. 1 shows a general depiction of such a system 10 and an externalprogramming device 70. The system 10 includes a number of wirelesselectrode assemblies 20, herein referred to simply as “seeds.” The seeds20 are implanted within chambers of the heart 30. In this example, thereare eight seeds 20, there being one implanted in the left atrium 32,three implanted in the left ventricle 34, one implanted in the rightatrium 36, and three implanted in the right ventricle 38. In oneembodiment, each of the seeds 20 has an internal coil that isinductively coupled with an external power source coil to charge anelectrical charge storage device contained within the seed 20, and alsohas a triggering mechanism to deliver stored electrical charge toadjacent heart tissue.

In another embodiment, one or more of the seeds has no energy storagedevice such as a battery or capacitor. In such a situation, each seedmay be comprised, for example, of a ferrite core having caps at each endwith ring electrodes encircling the caps, so as to form adumbbell-shaped configuration. A number of turns of fine insulated wiremay be wrapped around the central portion of the core so as to receiveenergy from a magnetic field produced by a shaped driving signal anddesigned to activate the electrodes. Such a configuration is discussedbelow in greater detail with reference to FIGS. 18A to 18C.

Referring again to FIG. 1, the system 10 also includes a pacingcontroller 40 and a transmitter 50 that drives an antenna 60 forcommunication with the seeds 20. Generally, the pacing controller 40includes circuitry to sense and analyze the heart's electrical activity,and to determine if and when a pacing electrical pulse needs to bedelivered and by which of the seeds 20. The sensing capability may bemade possible by having sense electrodes included within the physicalassembly of the pacing controller 40. Alternatively, a conventionalsingle or dual lead pacemaker (not shown in FIG. 1; although see FIG.2B) may sense the local cardiac electrocardiogram (ECG) and transmitthis information to antenna 60 for use by controller 40 in determinationof the timing of seed firing. In either case, the seed 20 need not beprovided with sensing capability, and also the seeds 20 need not beequipped with the capability of communicating to the pacing controller40 (for example, to communicate information about sensed electricalevents). In alternative embodiments, the seeds may communicate sensedinformation to each other and/or to the controller 40.

The transmitter 50—which is in communication with, and is controlled by,the pacing controller 40—drives an RE signal onto the antenna 60. In oneembodiment, the transmitter 50 provides both 1) a charging signal tocharge the electrical charge storage devices contained within the seeds20 by inductive coupling, and 2) an information signal, such as a pacingtrigger signal, that is communicated to a selected one or more of theseeds 20, commanding that seed to deliver its stored charge to theadjacent tissue.

An important parameter of the seed 20 that is a driver of the system 10design is the maximum energy required to pace the ventricle. This energyrequirement can include a typical value needed to pace ventricularmyocardium, but also can include a margin to account for degradation ofcontact between the electrodes and tissue over time. It is assumed thateach seed may require the maximum pacing threshold energy. Thisthreshold energy is supplied to the seeds between heartbeats by anexternal radio frequency generator (which may also be implanted), orother suitable energy source that may be implanted within the body.Typical values are:

Threshold pacing voltage=2.5 Volts

Typical lead impedance=600 Ohms

Typical pulse duration=0.4 mSec

Derived threshold energy=4 micro-Joules

Because RF fields at frequencies higher than about 100 kHz areattenuated by the body's electrical conductivity, and because electricfields of any frequency are attenuated within the body, energytransmission through the body may be accomplished via a magnetic fieldat about 20-100 kHz (or by a magnetic field pulse that contains majorfrequency components in this range), and preferably by transmission ofmagnetic fields in the range of 20-30 kHz when transmission is throughrelatively conductive blood and heart muscle.

As will be seen later in some of the specifically describedconfigurations of the system 10, the pacing controller 40 and thetransmitter 50 may be housed in a single enclosure that is bodyimplantable within a patient. In such a configuration, the singleenclosure device may have a single energy source (battery) that may beeither rechargeable or non-rechargeable. In another configuration, thepacing controller 40 and the transmitter 50 may be physically separatecomponents. As an example of such a configuration, the pacing controller50 may be implantable, for example in the conventional pacemakerconfiguration, whereas the transmitter 50 (along with the antenna 60)may be adapted to be worn externally, such as in a harness that is wornby the patient. In the latter example, the pacing controller 40 wouldhave its own energy source (battery), and that energy would not berechargeable given the relatively small energy requirements of thepacing controller 40 as compared to the energy requirements of thetransmitter 50 to be able to electrically charge the seeds 20. In thiscase, the pacing controller 40 would sense the local cardiac ECG signalthrough a conventional pacing lead, and transmit the sensed informationto the external controller. Again, transmission of information, asopposed to pacing energy, has a relatively low power requirement, so aconventional pacemaker enclosure and battery would suffice.

The external programmer 70 is used to communicate with the pacingcontroller 40, including after the pacing controller 40 has beenimplanted. The external programmer 70 may be used to program suchparameters as the timing of stimulation pulses in relation to certainsensed electrical activity of the heart, the energy level of stimulationpulses, the duration of stimulation pulse (that is, pulse width), etc.The programmer 70 includes an antenna 75 to communicate with the pacingcontroller 40, using, for example, RF signals. The implantable pacingcontroller 40 is accordingly equipped to communicate with the externalprogrammer 70, using, for example, RF signals. The antenna 60 may beused to provide such communications, or alternatively, the pacingcontroller 40 may have an additional antenna (not shown in FIG. 1) forexternal communications with the programmer 70, and in an embodimentwhere the transmitter 50 and antenna 60 are housed separately from thecontroller 40, for communications with the transmitter 50.

FIG. 2A shows an example system 200 of the type shown in FIG. 1. Thesystem 200 is shown as having been implanted in a patient, and inaddition, a programmer 270 is also shown that is external to thepatient. As shown, the system 200 is of a type that is entirelyimplantable. The system 200 includes several seed electrode assemblies220, there being four such assemblies shown as having been implantedwithin the heart 230 in FIG. 2A. The system 200 also includes animplantable combined pacing controller and transmitter device 240 thathas an antenna 260 for communicating, for example, to the seeds 220. Thecontroller/transmitter device 240 is shaped generally elongate andslightly curved so that it may be anchored between two ribs of thepatient, or possibly around two or more ribs. In one example, thecontroller/transmitter device 240 is 2 to 20 cm long and 1 to 10centimeters (cm) in diameter, preferably 5 to 10 cm long and 3 to 6 cmin diameter. Such a shape of the controller/transmitter device 240,which allows the device 240 to be anchored on the ribs, allows anenclosure that is larger and heavier than conventional pacemakers, andallows a larger battery having more stored energy. Other sizes andconfigurations may also be employed as is practical.

The antenna 260 in the FIG. 2A example is a loop antenna comprised of along wire whose two ends 270 and 272 extend out of the housing of thecontroller/transmitter device 240 at one end 280 of thecontroller/transmitter device 240. The opposite ends 270 and 272 of theloop antenna 260 are electrically connected across an electronic circuitcontained within the controller/transmitter device 240, which circuitdelivers pulses of RF current to the antenna, generating a magneticfield in the space around the antenna to charge the seeds, as well as RFcontrol magnetic field signals to command the seeds to discharge. Theloop antenna 260 may be made of a flexible conductive material so thatit may be manipulated by a physician during implantation into aconfiguration that achieves improved inductive coupling between theantenna 260 and the coils within the implanted seeds 220. In oneexample, the loop antenna 260 may be 2 to 22 cm tong, and 1 to 11 cmwide, preferably 5 to 11 cm long, and 3 to 7 cm wide. Placement of theantenna over the ribs allows a relatively large antenna to beconstructed that has improved efficiency in coupling RF energy to thepacing seeds.

In FIG. 2A, the loop antenna 260 has been configured to extend generallyaround the periphery of the housing of the controller/transmitter device240. In particular, the loop antenna 260 extends from its first end 270(located at the first end 280 of the controller/transmitter device 240)outwardly and then generally parallel to the elongately shapedcontroller/transmitter device 240 to the second end 282 of thecontroller/transmitter device 240. From there, the loop antenna 260extends outwardly and again generally parallel to thecontroller/transmitter device 240, albeit on an opposite side of thetransmitter/controller device 240, and back to the first end 280 of thecontroller/transmitter device 240. As such, the loop antenna 260 may,like the controller/transmitter device 240, be anchored to the ribs ofthe patient.

In this configuration, the distance between the center of the loopantenna 260 and the seed electrode assemblies 220 will typically be, onaverage, about three inches (3″). As will be shown later, such adistance puts significant power demands on the controller/transmitterdevice 240, and so an internal battery included within thecontroller/transmitter device 240 may need to be rechargeable. In someembodiments, however, the controller/transmitter device 240 may benon-rechargeable. The loop antenna 260 may have a shape that is morecomplex than that shown in FIG. 2, with a larger antenna area, ormultiple antenna lobes to capture more tissue volume. The antenna mayconsist of two or more wire loops, for example, one on the front of thepatient's rib cage, and a second on the back, to gain magnetic fieldaccess to a larger tissue region.

Referring to FIG. 2B, there is shown an embodiment as shown in FIG. 2A,but which also includes a conventional pacemaker, or pulse generator,290 and associated wired leads 295 which extend from the pulse generator290 and into chambers of the heart 600. As such, the pulse generator 290may be used to sense the internal ECG, and may also communicate with thecontroller/transmitter 240 as discussed previously.

Referring to FIG. 3, an embodiment of the controller/transmitter 240 andassociated loop antenna 260 is shown in block diagram form. Includedwithin the pacing controller 240 is: a battery 302, which may berecharged by receiving RF energy from a source outside the body viaantenna 260; ECG sensing electrodes 304 and associated sensing circuitry306; circuitry 308 for transmitting firing commands to the implantedseeds, transmitting status information to the external programmer,receiving control instructions from the external programmer andreceiving power to recharge the battery; and a controller or computer310 that is programmed to control the overall functioning of the pacingcontrol implant. In alternative embodiments, antenna 260 may receivesignals from the individual seeds 220 containing information regardingthe local ECG at the site of each seed, and/or antenna 260 may receivesignals from a more conventional implanted pacemaker regarding the ECGsignal at the sites of one or more conventional leads implanted on theright side of the heart.

FIG. 4 is a schematic diagram of an exemplary wireless electrodeassembly, or seed, 420 that may serve as the seeds 20 or 220 as shown ineither FIG. 1 or FIGS. 2A-B. The seed 420 includes, firstly, a receivercoil 410 that is capable of being inductively coupled to a magneticfield source generating a time-varying magnetic field at the location ofcoil 410, such as would be generated by the transmitter 50 and theantenna 60 shown in FIG. 1. The RF current in the external antenna maybe a pulsed alternating current (AC) or a pulsed DC current, and thusthe current induced through the receiver coil 410 would likewise be anAC or pulsed DC current. The current induced in coil 410 is proportionalto the time rate of change of the magnetic field generated at the siteof coil 410 by the external RF current source. A four-diode bridgerectifier 415 is connected across the receiver coil 410 to rectify theAC or pulsed DC current that is induced in the receiver coil 410. Athree-position switch device 418 is connected so that when the switchdevice 418 is in a first position, the rectifier 415 produces arectified output that is imposed across a capacitor 405. As such, whenthe switch device 418 is in the position 1 (as is the case in FIG. 4),the capacitor 405 stores the induced electrical energy.

The switch device 418, in this example, is a voltage-controlled deviceand is connected to sense a voltage across the capacitor 405 todetermine when the capacitor 405 has been sufficiently charged to aspecified pacing threshold voltage level. When the capacitor 405 issensed to have reached the specified pacing threshold level, thevoltage-controlled switch device 418 moves to a position 2, whichdisconnects the capacitor 405 from the coil 510. With the switch device418 in the position 2, the capacitor 405 is electrically isolated andremains charged, and thus is ready to be discharged. The voltagecontrolled switch device 418 may consist of a solid state switch, suchas a field effect transistor, with its gate connected to the output of avoltage comparator that compares the voltage on capacitor 405 to areference voltage. The reference voltage may be set at the factory, oradjusted remotely after implant via signals sent from the physicianprogrammer unit, received by coil 410 and processed by circuitry notshown in FIG. 4. Any electronic circuitry contained within the seed,including the voltage controlled switch, is constructed with componentsthat consume very little power, for example CMOS. Power for suchcircuitry is either taken from a micro-battery contained within theseed, or supplied by draining a small amount of charge from capacitor405.

A narrow band pass filter device 425 is also connected across thereceiver coil 410, as well as being connected to the three-positionswitch device 418. The band pass filter device 425 passes only a singlefrequency of communication signal that is induced in the coil 410. Thesingle frequency of the communication signal that is passed by thefilter device 425 is unique for the particular seed 20 as compared toother implanted seeds. When the receiver coil 410 receives a shortmagnetic field burst at this particular frequency, the filter device 425passes the voltage to the switch device 418, which in turn moves to aposition 3.

With the switch device in the position 3, the capacitor 405 is connectedin series through two bipolar electrodes 430 and 435, to the tissue tobe stimulated. As such, at least some of the charge that is stored onthe capacitor 405 is discharged through the tissue. When this happens,the tissue becomes electrically depolarized. In one example embodimentthat will be shown in more detail later, the bipolar electrodes 430 and435 across which stimulation pulses are provided are physically locatedat opposite ends of the seed 420. After a predetermined, or programmed,period of time, the switch returns to position 1 so the capacitor 405may be charged back up to the selected threshold level.

It should be noted that, for sake of clarity, the schematic diagram ofFIG. 4 shows only the seed electrical components for energy storage andswitching. Not shown are electronics to condition the pacing pulsedelivered to the tissues, which circuitry would be known to personsskilled in the art. Some aspects of the pulse, for example pulse widthand amplitude, may be remotely programmable via encoded signals receivedthrough the filter device 425 of the seed 420. In this regard, filter425 may be a simple band pass filter with a frequency unique to aparticular seed, and the incoming signal may be modulated withprogramming information. Alternatively, filter 425 may consist of anytype of demodulator or decoder that receives analog or digitalinformation induced by the external source in coil 410. The receivedinformation may contain a code unique to each seed to command dischargeof capacitor 405, along with more elaborate instructions controllingdischarge parameters such as threshold voltage for firing, duration andshape of the discharge pulse, etc.

Using seeds of the type shown in FIG. 4, all of the implanted seeds maybe charged simultaneously by a single burst of an RF charging field froma transmitter antenna 60. Because back reaction of the tiny seeds on theantenna 60 is small, transmitter 50 (FIG. 1) losses are primarily due toOhmic heating of the transmit antenna 60 during the transmit burst,Ohmic heating of the receive coil 410, and Ohmic heating of conductivebody tissues by eddy currents induced in these tissues by the applied RFmagnetic field. By way of comparison, if eight seeds are implanted andeach is addressed independently for charging, the transmitter 50 wouldbe turned ON eight times as long, requiring almost eight times moretransmit energy, the additional energy being primarily lost in heatingof the transmit antenna 60 and conductive body tissues. With the seed420 of FIG. 4, however, all implanted seeds are charged simultaneouslywith a burst of RF current in antenna 260, and antenna and body tissueheating occurs only during the time required for this single shortburst. Each seed is addressed independently through its filter device425 to trigger pacing. The transmitted trigger fields can be of muchsmaller amplitude, and therefore lose much less energy to Ohmic heating,than the transmitted charging pulse.

FIG. 5 is a flowchart of a pacing cycle that shows such a mode ofoperation of charging all implanted seeds 20 simultaneously, andtriggering the discharge of each seed 20 independently. The methodstarts at step 510 with the start of a charging pulse that charges allof the seeds simultaneously. When a pacing threshold voltage is attainedor exceeded, at step 520, the seeds switch to a standby mode (forexample, switch 418 in seed 420 moves to position 2). Next, in step 530,at the appropriate time, a controller/transmitter device such as device240 shown in FIG. 2, transmits a trigger pulse at a particular frequency(f1) that is passed through a band pass filter (such as filter device425) in the seed to be fired (for example, seed 1). Then, at step 540,that seed, namely seed 1, receives the trigger pulse through the bandpass filter, which in turn trips the switch to pace the tissue. Thisprocess may be repeated for each of the N number of seeds that have beenimplanted, as indicated at step 550, which returns to step 530 wherethere are additional seeds that have been charged and are to be fired.Next, at step 560 there is a delay until the next diastole, after whichtime the process begins anew at step 510. The exact time of firing ofthe first seed may be programmed by the physician in relation to the ECGsignal features measured by the sensing electrodes 304 in FIG. 3, or inrelation to ECG information transmitted to the controller 240 by thepacing seeds themselves, or in relation to pacing informationtransmitted to the controller 240 by a conventional implanted pacemaker,or in relation to pacing information received from a conventionalimplanted pacemaker through an implanted hard wire connection tocontroller 240. Subsequent timing of the firing of each additional seedmay be programmed by the physician at the time of implant. Note thatseeds may be programmed not to discharge. For example, an array of seedsmay be implanted, but only a subset may be programmed to receive firingcommands from the controller 240.

In the case of FIG. 2A and other similar embodiments, it is envisionedthat the controller/transmitter device 240 and associated antenna 260would first be implanted subcutaneously in a designed location (forexample, between the ribs in the case of the FIG. 2A embodiment). Thephysician then may program the controller/transmitter 240 by deliveringtelemetric signals through the skin using the programmer 270 in aconventional manner, although this programming may also be done, atleast in part, before implantation. One of the adjustable parameters isthe timing of firing of each seed 220, determined by a time at which ashort burst of current at the frequency for the particular seed 220 isdelivered to the antenna 260. The controller/transmitter device 240 mayhave a pair of sensing electrodes on its surface to detect thesubcutaneous electrocardiogram (ECG), or it may contain multipleelectrodes to provide a more detailed map of electrical activity fromthe heart. This local ECG signal sensed by the controller/transmitterdevice 240 may be used to trigger the onset of seed pacing when thepatient has a functioning sinus node. In any case, the signals sensed bythe controller/transmitter device 240 are used to monitor ECG signalsfrom the paced heart. In some cases, these ECG signals, or otherphysiologic sensor input signals, may be used to adjust or adapt thetiming of firing of the pacing seeds 220.

Alternatively, the controller 240 may receive local ECG or pacinginformation through an RF link from a conventional pacemaker 290implanted in the pectoral region of the patient, as shown in FIG. 28.This may be desirable in patients who already have a conventionalpacemaker, or when local ECG data from the conventional atrial or rightventricular apex pacing sites are desired to coordinate the timing offiring of the implanted seeds 220. Finally, the seeds 220 couldthemselves transmit information to controller 240 concerning the localbi-polar ECG measured at their sites. Alternatively, the seeds 220 couldsense the local ECG and discharge based upon this local data, with nofiring instructions from the controller 240 required, or the seeds 220could transmit information from seed 220 to seed concerning local ECGand onset of their discharge. All of the above embodiments, acombination, or a subset, may be implemented in this invention.

In an example embodiment, the seeds 220 would be delivered to theirrespective sites in the cardiac veins, within the heart walk or on theepicardial surface of the heart via a catheter, as will be described inmore detail later. A distal portion, or tip of the catheter, may containa single electrode or a pair of electrodes, each being connected to asignal recorder via leads extending to a proximal end of the catheter.As such, it is possible to obtain a unipolar or bipolar ECG at thecatheter distal tip. The physician would select the implantation sitebased upon features of the ECG signal sensed using the catheter. Theseed then may be injected through a needle extended from the cathetertip, or it may be pushed into the tissue and then released from thecatheter. Many mechanisms may be used for seed release, including therelease or addition of fluid pressure to the catheter tip.

Once implanted, the seed 220 may be charged and then fired to observethe altered electrogram proximate the seed at the location of thecatheter tip. The physician can adjust the timing of seed firing byprogramming the controller/transmitter device 240. When satisfied withthe local and controller/transmitter device 240 electrograms, thecatheter (or a seed delivery mechanism residing within the catheter) maybe removed, and a new delivery mechanism containing the next pacing seedmay be inserted and navigated to the next pacing site. Because seeds canbe fired in any order, or not fired at all, a physician may deliver theseeds in any order. When the heart is deemed to be beating in synchrony,no further seeds need be implanted. Alternatively, if it has beendetermined that the seeds are small enough that they do notsubstantially impair local tissue function, then an array of seeds maybe delivered to the veins and/or heart wall, and the physician canprogram a subset of seeds to fire in a sequence that optimizes thepumping efficiency of the heart. Ejection fraction and cardiac outputmay be measured to determine pumping efficiency. On any given heartbeat,some or all of the seeds would fire. The controller 240 may beprogrammed to sequentially fire seeds, or some seeds may firesimultaneously.

FIGS. 6-10 show an example of a mechanical design for a seed electrodeassembly and an example seed delivery device and method. Referring firstto FIG. 6, a system of the type shown in FIG. 2 is shown where threeseed electrode assemblies 220 have been implanted within tissue of theheart 600, and in particular, within a myocardial wall 605 of the heart600. In addition, the controller/transmitter device 240 is shownimplanted beneath the skin 610 of the patient. The antenna 260 extendsfrom within the controller/transmitter device 240 at one end of thedevice 240, and then extends around the periphery of the device 240, asdescribed previously. The external programming device 270 is also shown,which is used to communicate with the implanted controller/transmitter240.

Distal portions of two seed delivery catheters 615 are shown in FIG. 6,each extending within a chamber of the heart 600 and to a site nearwhere one of the seeds 220 is located. Generally, the delivery catheter615 enables placement of a seed 220 and the ability to sense theelectrical activity at the distal tip of delivery catheter 615 throughcatheter tip electrode 625, so that a physician can determine if thelocation is a good candidate location for implantation of seed 220. Ifthe location is a good candidate, the seed 220 may be partially insertedinto the tissue as shown in FIG. 9. With the seed 220 still tethered toa pull wire 735A, the seed 220 may be charged and then discharged intothe tissue, while the physician observes electrograms, including thelocal electrogram arising from electrode 625, and perhaps an electrogramfrom the distal seed electrode taken through the pull wire 735A. Uponfiring the seed, if the physician determines it is not in the properlocation to optimize cardiac output, then the seed 220 may be removedfrom that site and positioned elsewhere. If it is an appropriatelocation, then the seed 220 has an anchoring mechanism that can beactivated to implant the seed 220 permanently within the tissue so thatit retains its location.

Each of the catheters 615 is shown in FIG. 6 extending into the heart600 through a heart entry vessel 620 such as the inferior vena cava (forright chamber entry) or aortic valve (for left chamber entry). A distalportion 625 of the delivery catheter 615 includes a sensing electrodefor sensing the electrical activity at a tissue site where the seed 220may be implanted.

FIG. 7 shows one of many possible embodiments of a wireless electrodeassembly, or seed, 220. The seed 220 is shown in FIG. 7 within a distalportion of the seed delivery catheter 615. The seed 220 has a main body702 that, in this example, is bullet shaped and has two bipolarelectrodes 705 and 710. One of the electrodes, namely electrode 705, islocated at a distal tip of the bullet-shaped seed body 702, and theother electrode 710 is located at a proximal end of the seed body 702.The bullet shape of the seed body 702 enables it to be extended intotissue such as the myocardial wall 605, as will be illustrated in laterfigures. In other embodiments, the “nose,” or distal tip, of the seedbody 702 may be more cone-shaped than the embodiment shown in FIG. 7.While the distal and proximal electrodes 705 and 710 are shown on theseed itself, other locations are possible, including placing the distaland proximal electrodes 705 and 710 at the ends of the attachment tinesto achieve the maximum separation between electrodes.

The seed delivery catheter 615 consists of an elongate tube with a mainlumen 712 extending though its entire length. The catheter 615 has anopening 713 at its distal end so that the seed 220 may be released fromthe delivery catheter 615. The catheter 615 also has the previouslydiscussed electrode 625, which as shown extends around the periphery ofthe distal opening 713. An electrically conductive lead 716 is attachedto the electrode 625 and extends proximally through the entire length ofcatheter lumen 712, or through the wall of the catheter, and outside thebody (not shown in FIG. 7). The lead 716 is made of an electricallyconductive material, and thus provides the local electrocardiogram (ECG)appearing at the distal electrode 625. As such, the electrical activityappearing at the location of the distal seed electrode 705 may be viewedexternal of the patient to determine if that is an appropriate locationto implant the seed 220.

By way of example, the main lumen 712 of the seed delivery catheter 615may have an internal diameter of about two-and-a-half millimeters, andthe seed delivery catheter 615 may have an outside diameter that isslightly larger than that. In this case, the seed body 702 may have awidth of about two millimeters, and the length of the seed body 702 maybe about five to ten millimeters, for example. This enables the seed 220to be implanted entirely within a myocardial wall 605, which may, forexample, be about 20 millimeters thick in the left ventricle.

The seed 220 has a pair of forward-end tines 715 and 720 that eachextend from a common junction point 725. Each of the tines 715 and 720may be about three to eight millimeters in length, for example. The seedbody 702 also has a central bore 730 extending longitudinally through acenter of the seed body 702. In FIG. 7, which shows the seed 220 not yetimplanted, one of the forward-end tines, namely tine 720, extendsproximally into the bore 730, while the other forward-end tine 715extends distally to enable it to pierce through tissue. As will bedescribed in more detail later, the junction point 725 for the tines 715and 720 may be pushed forward of the seed 220 body, and when theconstrained tine 720 clears the central bore 730, the tines 720 and 715are biased to snap into a lateral configuration that will be shown in alater figure. The junction point 725 is physically larger than thediameter of the central bore 730, and thus enables the seed 220 to bepulled in a proximal direction by pulling on extraction wire 735.

The seed extraction wire 735 is attached to the junction point 725, andextends proximally through the entire length of the seed central bore730, and from there continues proximally through the delivery catheter615 and outside the body (not shown in FIG. 7). The wire 735 may be madeof an electrically conductive material so as to sense an electricalsignal appearing at a distal end of the wire 735, thus serving as anextraction pull wire and as a temporary ECG lead for distal electrode705. This is a means of sensing a bipolar electrocardiogram at aproposed implantation site before permanently implanting the seed 220,using electrode 705 (with wire lead 735) as a first electrode, and usingthe catheter electrode 625 and lead 716 as a second electrode.

In that the extraction wire 735 extends outside the patient's body, aphysician may pull the wire 735, and given that the junction point 725is too large to be pulled into the seed body central bore 730, pullingthe wire 735 pulls the seed 220 proximally within the delivery catheter615. The extraction wire 735 is also constructed of a material and of adiameter such that the wire 735 is rigid enough to be pushed forward toextend the junction point 725 forward of the seed 220 body and hencefree the forward-end tine 720 from the constraining central bore 730.The wire 735 has stopper device 740 that is attached to the wire 735 ata point that is proximal of the seed 220 body. The stopper device 740,like the junction point 725, is larger than the seed body central bore730, and thus constrains how far the lead junction point 725 can beextended forward of the seed body 702. The stopper device 740 ispositioned on the wire 735 at a location that is far enough away fromthe rear-end of the seed body 702 such that wire 735 may be pusheddistally far enough to free the constrained tine 720 from the seed bodycentral bore 730.

The extraction wire 735 has a detachment mechanism 745 located on thewire 735 at a point that is immediately distal of the stopper device740. The detachment mechanism 745 may be activated by a physician todetach the portion of wire 735 that is proximal of the detachmentmechanism 745. Various detachment mechanisms may be used for thedetachment mechanism 745. For example, the detachment mechanism 745 maybe a high-resistance portion of a conductive line that extendsproximally to a point external of the patient, and that can be heatedand detached by injecting current of a specified amount into theconductive line. In this case the wire 735 may serve three purposes:extraction of a seed 220 from a location that does not provide optimalcardiac resynchronization; conduction of the tip electrode 705 ECGsignal to a recorder outside the body; conduction of a burst of currentto detach itself at a point 745 of relatively high electricalresistance. Another example for the detachment mechanism 745 is amechanical configuration where the proximal detachable portion of thelead 735 may be unscrewed from the remainder of the lead 735, or wherethe lead 735 is pushed and turned in a certain way to effect detachmentof the proximal portion from the remainder of the lead 735. A mechanicalskiving or shearing means (not shown) may alternatively be applied atpoint 745.

The seed 220 also has a pair of tines 750 and 755 that extend from therear end of the seed body 702. In the shown example, there are two suchtines 750 and 755, though it will be understood that there may be morethan two tines, or a single tine. The tines 750 and 755 assist insecuring the seed 220 at a desired location within the tissue, such aswithin a desired location of the myocardial wall 605, to prevent theseed from migrating under the repeated stress of heart musclecontraction. The tines 750 and 755, in this example, are attached to therear-end electrode 710 near a periphery of the electrode 710, and extendfrom their attachment points in a direction that is about 45 degreesfrom a longitudinal axis of the seed body 702. As shown in FIG. 7,however, far ends of the tines 750 and 755 are constrained by an outerwall of the catheter lumen 712, and become bent toward the longitudinalaxis of the catheter 615. When the seed 220 is pushed out of the distalend of catheter 615, the tines 750 and 755 spring outwardly into theirnormal position (not shown in FIG. 7).

A tube 760 that is movable longitudinally within the catheter 615 isused to push the seed 220 distally within the catheter 615 and out ofthe catheter distal opening 713. The tube has a lumen 765 extendinglongitudinally through its entire length so that the wire 735 extendsthrough the tube lumen 765. The cross-sectional diameter of the pushertube 760 may be, for example, about half that of the catheter lumen 712.As such, where the catheter lumen 712 diameter is about 2.5 mm, the tubecross-sectional diameter may be about 1.25 mm.

In FIG. 8, the seed delivery catheter 615, with a seed 220 containedwithin, is shown with its circular distal electrode 625 pressed againstthe myocardial wall 605. In the configuration shown, it is possible forthe electrical activity occurring at that site of the myocardial wall605 to be monitored at a proximal end of the lead 716 to determine ifthe site is an appropriate candidate site in which to implant the seed220.

Turning now to FIG. 9, two seeds 220A and 220B are shown. The first seed220A is shown during the process of implanting the seed 220A within themyocardial wall 605, with the assistance of the seed delivery catheter615. The second seed 220B is shown as having already been permanentlyimplanted within the myocardial wall 605.

The first seed 220A is shown as having been pushed nearly entirelywithin the myocardial wall 605. This was accomplished by the physicianpushing the push tube 760 within the seed delivery catheter 615 so as topush the seed 220A out of the catheter's distal opening 713. Theforwardly extending distal tine 715 served to pierce the myocardial wall615 and permit implantation within the wall 615.

In the position shown in FIG. 9, the seed's rear-end tines 750A and 755Aare still partially within the seed delivery catheter 615 and thus arestill being constrained from extending outwardly from the seed body'slongitudinal axis. As such, it is still possible for the physician topull back the seed 220A from this position by pulling on the seedextraction wire 735A. If the seed 220A were to have been pushed a littlefurther so that the proximal tines 750A and 755A become extended, thenit may not be possible to pull back the seed 220A. As discussedpreviously, seed 220A may be charged and commanded to discharge whilewire 735 serves as a lead to monitor the electrical activity at theforward end of the seed 220A. The physician may determine that thepresent positioning is not appropriate, and wire 735 may then be pulledto extract the seed, which may then be moved to an alternate location.

Also in the position shown in FIG. 9, the wire 735 has not yet beenpushed forward to deploy the distal tines 715A and 720A (750A not shownin FIG. 9). Deploying the distal tines 715A and 720A is done as follows.First, the pushing tube 760 is used to push the seed 220A so that,firstly, the proximal tines 750A and 755A are freed from the deliverycatheter 615 and thus extend outwardly, and secondly, the seed's distaltine junction point 725A extends distally of the seed, and preferablyentirely through the myocardial wall 605. In particular, the junctionpoint 725A and one of the forward-end tines 715 are both positionedoutside the myocardial wall 605 in FIG. 9. Next, the wire 735A is pusheddistally until the lead stopper device 740 becomes flush with theproximal seed electrode 710A. When this occurs, the constrained tine720A becomes removed from the seed body central bore, thus allowing thetwo distal tines 715A and 720A to pop into the lateral position. Seed220B is shown in the deployed position, the proximal tines 750B and 755Bare shown extended, and the two distal tines 715B and 720B are outsidethe myocardial wall 605 and extend laterally from the junction point725B.

Referring now to FIG. 10, a flowchart is shown that describes a methodof delivering a seed 220 using the catheter 615 or another similardelivery device. The method begins at step 1010 with the percutaneoustransluminal delivery of the catheter 615 to the heart chamber. This maybe accomplished in the following manner. First, an introducer is used toprovide entry into, for example, the femoral vein or artery (dependingon where the seed 220 is to be delivered). The catheter 615 is theninserted so that its distal end is snaked through the inferior vena cavaand into the right atrium, for example. Thus, a seed 220 may bedelivered in the right atrium. The distal end of the catheter 615 mayalso be moved from the right atrium, through the tricuspid valve, andinto the right ventricle, for delivery of a seed 220 there. The distalend of the catheter may also be pushed through the fossa ovalis,accessed on the right atrial septum, for placement of seeds 220 in theleft heart chambers. Alternatively, the distal end of the catheter 615may be snaked through the femoral artery and descending aorta, throughthe aortic valve and into the left ventricle, and from the leftventricle may be moved through the mitral valve into the left atrium.Navigating the catheter 615 may require that the catheter 615 have sometype of navigational capability such as push and pull wires commonlyused with electrophysiology catheters.

Next, at step 1020, a sample ECG signal may be taken at sites on theheart inner wall. This may be done with the catheter 615 positioned asshown in FIG. 8, for example. At step 1030, the physician selects a siteat which to deliver the seed 220. Then, at step 1040, the physiciandelivers the seed 220 into the myocardial wall tissue, such as shownwith seed 220A in FIG. 9. At this point, the seed 220 is still tetheredby the lead 735A so that the seed may be pulled back into the deliverycatheter 615 if necessary. Further at step 1040 a test pace is performedto test the response at this site. This may be done using the programmer270 shown in FIG. 6 to instruct the controller/transmitter device 240 tosend a charging signal and then a trigger signal to the particular seed220.

If the pacing response is found, at step 1050, to be unacceptable, thenthe seed 220 may be removed and the process may be performed againstarting at step 1020. If, on the other hand, the pacing response isfound to be acceptable, then, at step 1060, the anchoring means for theseed 220 may be activated, for example, by moving the seed 220 entirelyout of the catheter 615 and freeing the proximal tines 750 and 755 fromthe constraints of the catheter 615 and pushing the lead 735 to releasethe distal tines 715 and 720. Also at step 1060, the tether to the seed220 may be released, for example, using the detachment mechanism 745.Having completed the implantation of the seed, it is now possible atstep 1070 to begin placement of the next seed 220.

As discussed previously, each of the seeds 220 may have a filter 425(see FIG. 4) that allows passage of a signal of a particular frequency.Thus, for example, where eight seeds 220 are implanted, each of theseeds 220 may have a band pass filter 425 of a different centerfrequency. To make this possible, seeds 220 may be manufactured ashaving one of sixteen different band pass frequencies. Thus, up tosixteen seeds 220 may be implanted so that each seed is separatelycontrollable. A code for the particular pass frequency may be labeleddirectly on the seed 220 itself, or alternatively, may be labeled on thepackaging for the seed 220. As such, when programming the system 200using the programmer 270, the particular band pass frequency for eachseed 220 is communicated to the pacing controller 240.

A variety of alternative embodiments are envisioned for seed deliveryand detachment. For example, FIG. 11A shows a seed 1120A that is securedinto the myocardium 605 with a distal spring 1105A, or “cork screw.” Adelivery rod 1110 provided by a delivery catheter 1112 is detached fromthe seed 1120A by turning the rod 1110 to engage the spring into tissueand also unscrew the threaded distal rod section 1115 from the seed1120A. In FIG. 11B, a distal spring 1105B is screwed into the myocardium605 using a clockwise rotation of the seed 1120B, which also unscrewsthe delivery rod from the seed. Upon removal of the delivery rod,proximal spring 1125 is exposed to the myocardium 605. Clockwise spring1105B and counter-clockwise spring 1125 together prevent rotation andtranslation of the seed through the myocardium. A mechanism for releaseof the springs is not shown in the figure. A small push rod passingthrough the delivery rod and seed could be used to push the distalspring from the seed and into a locked position. A thin sheath couldcover proximal spring 1125. The thin sheath would be retracted alongwith the delivery rod. Alternate means for detachment of the deliveryrod include Ohmic heating of a high resistance portion of the rod, andmechanical shearing. In FIG. 11C-D, tines 1130 are pushed, using a pushrod 1135 provided through the main lumen of the delivery catheter 1112,from the central portion of the seed 1120C, out through channels 1140and into the myocardium 605, so that the tines 1130 extend laterallyfrom the seed 1120C body (as shown in FIG. 11D), and so that the seed1120C becomes secured within the tissue. The push rod 1135 is removable,at an attachment point, from a proximal end junction point 1145 of thetines 1130. Various mechanisms for removing, or detaching the push rod1135 from the tine proximal end junction point 1145 may be employed, asdiscussed previously in connection with the FIG. 7 embodiment.

Referring now to FIGS. 11E-K, some embodiments that are envisioned forseed delivery and detachment include a seed 1120E having a helical tine1105E and one or more adjustable tines 1110E that secure the seed 1120Eto the myocardium 605. In such embodiments, detachment mechanisms 1145Eand 1165E may be used to release the seed 1120E from an elongate shaft1160E after the seed 1120E is secured to the myocardium 605.

Referring to FIG. 11E, the seed 1120E is shown within a distal portionof the seed delivery catheter 615. The seed 1120E has a main body 1122Ethat, in this example, is cylindrically shaped with a tip portion 1123Eat a distal end. The seed 1120E may include two bipolar electrodes 1135Eand 1136E that are capable of discharging an electrical pulse. Electrode1135E is located at the distal end of seed body 1122E, and the otherelectrode 1136E is located at a proximal end of the seed body 1122E. Inthis embodiment, the tip portion 1123E of the seed body 1122E has amodified cone shape that facilitates delivery of the distal end of theseed 1120E into tissue such as the myocardial wall 605, as will beillustrated in later figures. The tip portion 1123E may serve as astrain relief mechanism for the adjustable tines 1110E that extend fromthe tip portion 1123E. Furthermore, the tip portion 1123E may alsodeliver a steroid elution to minimize the formation of fibrous tissue atthe seed/myocardium interface. While the distal and proximal electrodes1135E and 1136E are shown on the seed body itself, other locations arepossible. For example, the distal electrode 1135E may be disposed at theend of the helical tine 1105E to achieve the maximum separation betweenelectrodes, or may be an entire tine. In another example, the surface oftip portion 1123E on the seed body 1122E may function as the distalelectrode 1135E, which may provide a more efficient use of space whenthe seed body 1122E is substantially smaller in size. Furthermore, usingthe surface of tip portion 1123E to function as the distal electrode1135E may be desirable in circumstances where only the tip portion 1123Econtacts the endocardium or myocardium tissue (described in more detailbelow).

As previously described, the seed delivery catheter 615 includes anelongate tube with a main lumen 712 extending though its entire length.The catheter 615 has an opening 713 at its distal end so that the seed1120E may be released from the distal end of the delivery catheter 615.In some circumstances, all or a portion of the seed 1120E may extendfrom the delivery catheter 615 before the seed 1120E is secured to theheart tissue. In those cases, the main lumen 712 may still be sized toslidably engage the elongate shaft. The catheter 615 may also have anelectrically conductive lead 716 and an electrode 625 that extendsaround the periphery of the distal opening 713 and is capable ofproviding local ECG information as previously described. In someembodiments, it may be necessary to secure the tip of the catheter 615to the heart tissue during seed placement. For example, the distal endof the catheter 615 may include a screw mechanism to temporarily securethe catheter 615 to the heart tissue (described in more detain below inconnection with FIG. 13).

In this embodiment, the seed 1120E has a plurality of adjustable tines1110E that each extend from a common junction member 1112E. As shown inFIG. 11E, each of the adjustable tines 1110E generally extend from thejunction member 1112E through a central bore 1130E of the seed body1122E. FIG. 11E shows the seed 1120E not yet implanted, and only thehelical tine 1105E extends from the seed body 1122E while the adjustabletines 1110E are disposed in the central bore 1130E. As will be describedin more detail later, the junction member 1112E may be pushed in adistal direction by an actuation rod 1170E, thereby forcing theadjustable tines 1110E from the distal end of the central bore 1130E.When the constrained tines 1110E extend from the central bore 1130E, thetines 1110E are biased to extend in a curled or hook configuration. Thejunction member 1112E may be physically larger than the diameter of thecentral bore 1130E, providing a stopping point for actuation of theadjustable tines 1110E.

Still referring to FIG. 11E, the elongate shaft 1160E includes adetachment mechanism 1165E at a distal end that is capable ofengaging/disengaging the detachment mechanism 1145E of the seed 1120E.In this embodiment, the detachment mechanism 1165E includes a threadedmember that engages a complementary threaded member on the seed'sdetachment mechanism 1145E. The threaded engagement between thedetachment mechanisms 1165E and 1145E may be arranged so that thethreads would not release when the seed 1120E is being advanced into thetissue with the rotation of the helical tine 1105E.

From the detachment mechanism 1165E, the elongate shaft 1160E continuesproximally through the delivery catheter 615 and outside the patient'sbody (not shown in FIG. 11E). In that the elongate shaft 1160E extendsoutside the patient's body, a physician may direct the seed body 1122E(via the elongate shaft 1160E coupled thereto) through the lumen 712 ofthe delivery catheter 615. (As described in more detail below inconnection with FIG. 11I, the delivery catheter 615 may be navigatedthrough an access catheter or other steerable sheath to the implantationsite. The access catheter is capable of maintaining a stable valvecrossing, which can reduce trauma to the valve and facilitate theimplantation of multiple seeds into the wall of the heart chamber.) Theelongate shaft 1160E may be constructed of a material and of a size anddesign such that the elongate shaft 1160E is sufficiently rigid to berotated within the main lumen for purposes of engaging the helical tine1105E with the myocardium tissue. Also, the elongate shaft 1160E may besufficiently flexible so as to not impede navigation of the elongateshaft 1160E and the catheter 615 to the implantation site.

The actuation rod 1170E may be disposed in a lumen 1162E of the elongateshaft 1160E. The actuation rod 1170E includes an engagement surface1172E that is adapted to contact the junction member 1112E. From theengagement surface 1172E, the actuation rod 1170E may continueproximally through the elongate shaft 1160E and outside the patient'sbody. In such embodiments, a physician may apply a force at the proximalend of the actuation rod 1170E so as to slide the rod 1170E within theelongate shaft 1160E. Such motion of the elongate rod 1170E may apply adistal force upon the junction member 1112E. The actuation rod 1170E maybe constructed of a material and be of a size such that the actuationrod is sufficiently rigid to push against the junction member 1112E andforce adjustable tines 1110E to extend from the distal end of thecentral bore 1130E. Also, the elongate rod 1170E may be sufficientlyflexible so as to be guided through the lumen 1162E of the elongateshall 1160E.

Referring now to FIGS. 11F-11H, at least a portion of the seed 1120Eshown in FIG. 11E may be implanted into myocardium 605. As previouslydescribed in connection with FIG. 6, the delivery catheter 615 may beguided into a heart chamber (e.g., left atrium 32, left ventricle 34,right atrium 36, or right ventricle 38) to enable placement of at leasta portion of the seed 1120E from the heart chamber into the myocardium605. In such circumstances, the seed may pass necessarily from thedistal opening 713 of the catheter 615, through an inner lining of theheart wall (e.g., the endocardium 606), and into the myocardium 605.FIGS. 11F-11H show a seed 1120E that is being implanted into themyocardium 605 and also show a neighboring seed 1120E (below the firstseed 1120E) that was previously secured to the myocardium 605.

Referring to FIG. 11F, the seed 1120E in the lumen 712 of the deliverycatheter 615 may be directed toward the distal end by a force 1167E fromthe elongate shaft 1160. The distal end of the delivery catheter 615 mayabut (or be positioned proximate to) the inner surface of the heartchamber so that the seed 1120E is guided to a selected site of the heartwall. As shown in FIG. 11E, adjustable tines 1110E of the seed 1120E inthe delivery catheter 615 are not in an actuated position where theyextend from the distal end of the central bore 1130E (the adjustabletines 1110E of the neighboring seed 1120E that was previously implantedare shown in an actuated position). The helical tine 1105E is configuredto penetrate through the endocardium 606 and into the myocardium 605, asdescribed in more detail below.

Referring to FIG. 11G, the seed 1120E in the lumen 712 of the deliverycatheter 615 may be rotated by a torsional force 1168E from the elongateshaft 1160. By rotating the seed body 1122E along its longitudinal axis,the helical tine 1105E may be “screwed” into the heart wall. In suchcircumstances, the helical tine 1105E penetrates through the endocardium606 and into the myocardium 605. In some embodiments where thedetachment mechanism 1145E includes a threaded member, the torsion force1168E from the elongate shaft 1160E may serve to maintain or tighten thethreaded engagement.

In the position shown in FIG. 11G, the seed's adjustable tines 1110E arenot extended from the central bore 1130E (as shown by the neighboringseed). As such, it is still possible for the physician to pull back theseed 1110E from this position by rotating the elongate shaft 1160E in adirection opposite of force 1168E, which would cause the helical tine1105E to “unscrew” from the myocardium tissue. The seed's distalelectrode 1135E is in contact with the myocardium 605. As discussedpreviously, seed 1120E may be commanded to discharge a pacing electricalpulse while electrode 625 on the delivery catheter 615 monitors theelectrical activity at the selected site. If the physician determinesthat the present positioning of the seed 1120E is not satisfactory, theseed 1120E may be retracted into the delivery catheter lumen 712, whichmay then be moved to an alternate location. At the alternate location,the helical tine 1105E would again penetrate through the endocardium andinto the myocardium 605, in which case further monitoring of electricalactivity may occur.

Referring to FIG. 11H, after the seed 1120E is secured to the heart wall(e.g., at least a portion of the helical tine 1105E and perhaps aportion of the seed body 1122E is penetrated into the endocardium) andafter the physician determines that the positioning of the seed 1120E isproper, the adjustable tines 1110E may be forced to an actuatedposition. In this embodiment, the actuation rod 1170E disposed in theelongated shaft 1160E is capable of applying a force on the junctionmember 1112E. When the junction member 1112E is forced toward the seedbody 1122E, the adjustable tines 1110E extend from the distal end of thecentral bore 1130E. In this embodiment, the adjustable tines 1110E arebiased to have a curled or hook shape when unconstrained by the centralbore 1130E. For example, the adjustable tines 1110E may comprise a shapememory alloy material, such as nitinol or the like, that is capable ofreturning to its biased shape after being elastically deformed withinthe central bore 1130E. The adjustable tines 1110E embed in themyocardium 605 to provide supplemental anchoring support and tosubstantially hinder additional rotation of the seed body 1122E. Assuch, the elongate shaft 1160E may be rotated backward relative to theseed body 1122E, which causes the threaded members of detachmentmechanisms 1165E and 1145E to disengage one another. In this embodiment,the elongate shaft 1160E may be rotated relative to the seed body 1122Ewithout extracting the seed 1120E from the myocardium 605 because theadjustable tines 1110E prevent the helical tine 1105E from being“unscrewed.” After the seed 1120E is detached from the elongate shaft1160E, the delivery catheter 615 and the elongate shaft 1160E may bewithdrawn from the implantation site.

In addition to preventing the seed body 1122E from substantiallyrotating within the myocardium 605, the adjustable tines also reduce thelikelihood of the seed body 1122E being pulled or torn from the heartwall. The seed 1120E may be exposed to various forces from the beatingheart and the turbulence of the blood in the heart chambers. In someembodiments, the seed 1120E may be attached to the heart wall so that athreshold amount of pull force is required to remove the seed 1120E fromthe heart wall. Certain embodiments of seed 1120E may be secured to theheart wall such that a pull force of greater than 0.3 lbs. is requiredto remove the seed body 1122E from the heart wall. In some embodiments,the a seed 1120E may be secured to the heart wall such that a pull threeof greater than 0.5 lbs., and preferably greater than 1.0 lbs., isrequired to remove the seed body 1122E from the heart wall.

In one example, several seeds 1120E were secured to the myocardium of aporcine (pig) heart using the helical tine 1105E and three adjustabletines 1110E. The porcine heart was delivered to a lab where a portion ofit was removed by scalpel to reveal an internal heart chamber. Severalseeds 1120E were secured to the porcine heart wall from the internalheart chamber-first by rotating the helical tine 1105E into themyocardium and then by actuating the adjustable tines 1110E to a curledshape substantially within the myocardium tissue. Each of the seeds1120E was secured to the heart wall such that a pull force of greaterthan 0.3 lbs. was required to remove the seed body 1122E from the heartwall, and in some instances, a pull force of greater than 1.0 lbs. wasrequired.

Referring now to FIG. 11I, helical tine 1105E and the adjustable 1110Emay secure the seed 1120E to the myocardium 605 such that at least aportion of the seed body 1122E (e.g., the tip portion 1123E) penetratesinto the myocardium 605. In some embodiments where the seed 1120E issubstantially smaller than the myocardium thickness, the seed body 1122Emay be fully inserted into the myocardium tissue. In the embodimentsdescribed in connection with FIGS. 11F-11H, a distal portion of the seedbody 1122E extends into the myocardium 605 white a proximal portion ofthe seed body 1122E is exposed to the heart chamber (e.g., left atrium32, left ventricle 34, right atrium 36, or right ventricle 38). As shownin those figures and in FIG. 11I, the seed body 1122E may be secured tothe myocardium 605 so that the distal electrode 1135E is in contact withthe myocardium white the proximal electrode 1136E is exposed to theheart chamber and the blood therein). In certain cases, such positioningof the seed body 1122E may be dictated by a limited thickness in themyocardium wall.

Still referring to FIG. 11I, in some cases the seed body 1122E may notfully penetrate into the myocardium 605. For example, as shown by thelower seed 1120E secured in the left ventricle 34 shown in FIG. 11, aportion of the seed 1120E (e.g., the helical tine 1105E and theadjustable tines 1110E) may penetrate through the endocardium while thea substantial portion of the seed body 1122E does not fully penetrateinto the myocardium tissue. In such circumstances, the tip portion 1123Emay contact or penetrate into the endocardium (and perhaps partiallyinto the myocardium), but the other portions of the seed body 1122E maynot penetrate into the heart wall. Yet in this position, the seed 1120Emay be capable of providing a pacing electrical pulse to the proximalheart tissue. The delivery of the pacing electrical pulse may befacilitated by using a surface of tip portion 1123E to function as thedistal electrode 1135E.

In some cases, such positioning of the seed body 1122E may provideoperational advantages. For example, if the distal electrode 1135E is acathode that generally depolarizes nearby tissue cells, and if theproximal electrode 1136E is an anode that may hyper-polarize nearbytissue cells, the position of the seed body 1122E shown in FIGS. 11F-11Imay reduce the effects of hyper-polarization. Because, in this example,the anode is generally exposed to blood in the heart chamber, the tissuecells in the myocardium are not necessarily hyper-polarized by theanode. In such circumstances, the pacing electrical charge between thecathode, the nearby myocardium, the nearby blood in the heart chamber,and the anode may reduce the hyper-polarization of local areas in themyocardium tissue—a factor that may limit pacing effectiveness.

Still referring to FIG. 11I, a distal end 676 of an access catheter 675may be guided to a heart chamber where the seed 1120E is to bedelivered. The access catheter 675 includes a lumen that extends from aproximal end to the distal end 676. The access catheter also includes adistal opening through which the delivery catheter 615 slidably passesas it is directed to the selected site proximal to the heart wall. Insome embodiments, the access catheter 675 may be used to establish andmaintain a valve crossing. In such circumstances, the delivery catheter615 may be fully withdrawn from the patient's body after a first seed1120E has been successfully implanted, yet the access catheter 675 canmaintain its position in the heart chamber. Then, anew delivery catheter615 and elongated shaft 1160E (with a second seed 1120E attachedthereto) may be guided through the access catheter 675 are into theheart chamber. As shown in FIG. 11I, the access catheter 675 mayapproach the left ventricle 34 through the aorta (e.g., across theaortic valve and into the left ventricle 34). Other approaches arecontemplated, depending on the targeted heart chamber, the conditions inthe patient's heart vessels, the entry point into the patient's body,and other factors. For example, the access catheter 675 may approach theleft ventricle 34 through the interior vena cava, through a puncture inthe atrial septum, and down through the mitral valve into the leftventricle 34.

As previously described, the delivery catheter 615 may include asteering mechanism, such as push or pull wires, to aid in placement ofthe distal end of the catheter 615 against a selected site on the wallof the heart. Similarly, the access catheter 675 may include a steeringmechanism, such as push or pull wires, to aid in placement of the distalend 676 in the selected heart chamber. In this embodiment, the accesscatheter 675 includes an image device 685, such as an ultrasound probeor the like, proximal to the distal end 676 of the access catheter 675.The image device 685 is capable of providing the physician withvisualization of the implantation site in the heart chamber. Because theinner surface of the heart chambers may be substantially irregular insurface topology as well as thickness, the image device 685 can be usedby a physician to visualize the implantation site and possibly measurethe myocardium wall thickness at that site. Such a feature may beparticularly advantageous where the procedure is to be conducted on anactive, beating heart.

Referring now to FIGS. 11J-11K, the adjustable tines 1110E of the seed1120E may be forced from a non-actuated position (e.g., FIG. 11J) to anactuated position (e.g., FIG. 11K). As previously described, the seed1120E may include a plurality of adjustable tines 1110E. In thisembodiment, the seed 1120E includes three adjustable tines 1110E thateach extend from the common junction member 1112E. As shown in FIG. 11J,when the adjustable tines 1110E are in a non-actuated position, thejunction member 1112E is offset from the seed body 1122E, and at least aportion of the adjustable tines 1110E are constrained in the centralbore 1130E. When the junction member 1112E is forced in a generallydistal direction toward the seed body 1122E, as shown in FIG. 11K, theadjustable tines 1110E are moved to an actuated position. As previouslydescribed, each of the tines 1110E may be biased to extend in a curledor hooked shape after being released from the central bore 1130E.

Referring now to FIGS. 11L-11N, alternate embodiments of the seed mayinclude adjustable tines that are not disposed in a central bore of theseed body. For example, some embodiments of a seed 1120L may include aplurality of adjustable tines 1110L that are disposed in non-centralbores 1130L that extend in a longitudinal direction near the peripheryof the seed body 1122L. The adjustable tines 1110L of the seed 1120L maybe forced from a non-actuated position (e.g., 110 to an actuatedposition (e.g., FIG. 11M). In this embodiment, the seed 1120L includes ahelical tine 1105L that extends distally from the seed body 1122L andincludes three adjustable tines 1110L that each extend from a commonjunction member 1112L. As shown in FIGS. 11M and 11N, tines 1110Linclude a distal tip 1121. As shown in FIG. 11J, when the adjustabletines 1110L are in a non-actuated position, the junction member 1112L isoffset from the seed body 1122L, and at least a portion of theadjustable tines 1110L are constrained in the associated peripheralbores 11301L. When the junction member 1112L is forced in a generallydistal direction toward the seed body 1122L, as shown in FIG. 11K, theadjustable tines 1110L are moved to an actuated position. As previouslydescribed, each of the tines 1110L may be biased to extend in a curledor hook shape after being released from its associated bore 1130L. Thetines 1110L may also extend from the sides of seed 1120L, such asthrough electrode 1135L, and could also operate to extend excitationsignals from electrode 1135L into the tissue.

Referring to FIG. 11N this embodiment of the seed 1120L may be directedto the targeted site of the heart wall using a delivery catheter 615 andan elongate shaft 1160L. The elongated shaft 1160L may include adetachment mechanism 1165L that engages/disengages with the seed 1120L.In this embodiment, the detachment mechanism 1165L includes a threadedmember that engages a complementary threaded member of the seed'sdetachment mechanism 1145L. As previously described, the seed 1120L maybe rotated such that the helical tine 1105L penetrates through theendocardium 606 and into the myocardium 605. When the seed 1120L isproperly positioned, a force from an actuation rod 1170L may move thejunction member 1112L in a distal direction toward the seed body 1122L.Such motion causes the adjustable tines 1110L to extend from the distalends of the peripheral bores 1130L, thereby causing the adjustable tines1110L and the helical tine 1105L to secure the seed 1120L to themyocardium 605. After the adjustable tines 1110L are moved to theactuated position, the elongate shaft 1160L may be rotated to releasethe seed 1120L at the detachment mechanisms 1145L and 1165L, whichpermits the delivery catheter 615 and the elongated shaft 1160L to bewithdrawn from the implantation site.

As previously described, the seed body may be secured to the hearttissue using tines, screws, barbs, hooks, or other fasteners. FIGS.11P-11U illustrate further examples of such attachment mechanisms.Referring to FIG. 11P, some embodiments of a seed 1120P may include abody screw 1106P and adjustable tines 1110P to secure the seed 1120P tothe myocardium 605. The body screw 1106P may include threads that arewound around the seed body 1122P so that rotation of the seed body 1122Pcauses that penetration through the endocardium 606 and into themyocardium 605. The threads may be interrupted and twisted in somecircumstances to help ensure that the seed 1120P does not back out ofthe tissue.

The adjustable tines 1110P may be actuated when a junction member 1112Pis moved in a distal direction toward the seed body 1122P. Referring toFIG. 11Q, some embodiments of a seed may include a single adjustabletine that helps to secure the seed to the myocardium 605. For example,the seed 1120Q may include a body screw 1106Q and an adjustable tine1110Q that is actuated by moving a junction member 1112Q toward the seedbody 1122Q.

The embodiment of FIGS. 11P-11Q may provide additional benefits toadvancing the seed 1120P into tissue. By providing a more tapered end onthe seed body 1122P and connecting the body screw 1106Q to the seed body1122P, the seed 1120P may create an opening for the passage of the seedbody 1122P more easily into the tissue, in some cases where the bodyscrew 1106Q is not used, the distal portion of the helical tine can passinto the heart wall tissue, but further progress may be blocked when theseed body 1122P abuts the tissue. Also, while the thread is shown inFIGS. 11P-11Q as being disposed tight to the seed body 1122P, it couldalso be separated slightly from the seed body 1122P, particularly aroundthe front tapered portion of the seed body 1122P, and then connectedback to the seed body 1122P, for example, by a thin webbed section thatcan itself cut into the tissue. While it is not necessary for allembodiments that the seed body be placed into the tissue, otherappropriate arrangements may be used that allow the seed body 1122 toenter into the tissue without significant disruption to the physicalstructure of the tissue.

Referring to FIG. 11R, some embodiments of a seed may include anadjustable barb that helps to secure the seed to the myocardium 605. Theadjustable barb may include biased extensions that outwardly shift whenno longer constrained in a bore. For example, the seed 1120R may includea body screw 1106R that transitions into a helical tine 1105R and anadjustable barb 1111R that is actuated by moving a junction member 1112Rtoward the seed body 1122R. Referring to FIG. 11S, some embodiments of aseed 1120S may include a helical tine 1105S and an adjustable barb 1111Sto secure the seed 1120S to the myocardium 605. The adjustable barb1111S may be actuated by moving a junction member 11125 toward the seedbody 1122S. Referring to FIG. 11T, some embodiments of a seed mayinclude one or more body barbs 1107T that help to secure the seed to themyocardium 605. The body barbs 1107T may extend from the seed body 1122Tand acts as hooks that prevent the retraction from the myocardium 605.For example, the seed 1120T may be fully embedded in the myocardium 605and include body barbs 1107T and adjustable tines 1110T that can beactuated by moving a junction member 1112T toward the seed body 1122T.Referring to FIG. 11U, some embodiments of a seed 1120U may include bodybarbs 1107U and an adjustable barb 1111U to secure the seed 1120U to themyocardium 605. The adjustable barb 1111U may be actuated by moving ajunction member 1112U toward the seed body 1122U.

Referring now to FIGS. 11V-11W, some embodiments of the detachmentmechanism between the elongate shaft and the seed may include a lockingmember that is movable between an engaged position (e.g., FIG. 11V) anda disengaged position (e.g., FIG. 11W). In such embodiments, theelongate shaft may have a noncircular outer cross-section (such as asquare or hexagonal cross-sectional outer shape) to facilitatetranslation of rotational motion to the seed body.

Referring to FIG. 11V, the seed 1120V may include a body 1122V andelectrodes 1135V and 1136V, as described in previous embodiments.Furthermore, the seed 1120V may include tines, screws, barbs, hooks, orother fasteners (such as a helical tine 1105V, adjustable tines 1110Vthat extend from a common junction member 1112V) as previouslydescribed. Also as previously described, the seed 1120V may be directedby an elongated shaft 1160V through a lumen 712 of a delivery catheter615. The seed 1120V may include a detachment mechanism 1145V having acavity 1146V shaped to receive at least a portion of a locking member1166V. In the depicted embodiment, the cavity 1146V may be curved to fita spherically shaped locking member 1166V like a small ball such that,when the locking member 1166V is engaged with the cavity 1146V, theelongate shaft 1160V is not retractable from the seed body 1122V.

Referring to FIG. 11W, when at least a portion of the seed 1120V isproperly positioned in the myocardium 605, a force 1177V may be appliedfrom the actuation rod 1170V may be to move the junction member 1112Vtoward the seed body 1122V. Such motion of the junction member 1112V maycause the adjustable tines 1110V to extend from the seed body 1122V,thereby securing the seed 1120V to the myocardium 605. In addition, themotion of the actuation rod 1170V may cause the locking member to moveto a disengaged position. For example, the actuation rod 1170V mayinclude a depressed surface 1176V that is substantially aligned with thelocking member 1166V when the actuation rod 1170V forces the junctionmember 1112V to actuate the tines 1110V. As such, the locking member1166V moves toward the depressed surface 1176V and disengages with thecavity 1146V. This disengagement permits the actuation rod 1170V, theelongate shaft 1160V, and the delivery catheter 615 to be withdrawn fromthe seed implantation site while at least a portion of the seed 1120Vremains secured to the myocardium 605.

Detachment mechanisms other than those discussed above may also be usedin appropriate situations. For example, multiple spherically shapedlocking members like that discussed above may be attached along thelength of a wire, such as by soldering. The wire may be passed down aninterior passage of multiple seeds that are mounted end-to-end on thetip of a catheter. Each locking member may be located so as to extendout of a central bore inside the seeds to lock against a correspondingcavity on an internal surface of a seed. In operation, and with lockingmember holding each seed in place, the most distal seed may be driveninto the tissue by rotating the seeds. The wire may then be withdrawnproximally the length of one seed, on that the locking member in themost distal seed is pulled back to the second-most-distal seed, and theother locking members move back one seed. Such a controlled withdrawalof the wire may be accomplished, for example, using an indexed triggermechanism that is handled by the surgeon. The second seed—now the mostdistal seed—may then be implanted, and the wire withdrawn again. In sucha manner, multiple seeds may be implanted from a single introduction ofthe mechanism into a heart chamber.

In addition, the seeds may be provided with alternative mechanisms forremoval, such as for use when the primary attachment mechanisms aredamaged, occluded, or otherwise unavailable. For example, severalchannels may be formed about the periphery of a proximal, nonimplantedelectrode. The channels may proceed from shallow to deep on that, forexample, a tool having radially-arranged fingers with inward extensionsmay position those extensions around the electrode. The fingers can thenbe contracted, such as by a sleeve that is slid down around the exteriorof the fingers, and the extensions may be received into the channels.The tool may then be rotated so that the extensions move down into thedeep portions of the channels and engage the seed in rotation so thatthe seed may be removed from the tissue.

FIG. 12 illustrates the possibility that seeds 1220 may be placedparallel to the heart wall 605, in addition or in preference totransverse placement. This may be particularly necessary where the heartwall is thin, for example in the atria or in regions of the ventriclesthat contain scar tissue. Placement parallel to the wall is particularlyrequired when the wall thickness is less than the seed length. Note thatthe catheter 1212 may be curved near its tip to facilitate parallelplacement. Since the heart wall 605 is moving during the cardiac cycle,it may be necessary to secure the tip of the catheter 1212 to the hearttissue during seed placement. This concept is illustrated in FIG. 13,showing a cork screw 1350 temporary securement of the catheter 1312 tothe wall 605. Tines that extend from the distal end of the catheter forpenetration into the heart wall to secure and stabilize the catheter tipduring seed delivery are also envisioned. The tines would be extendedinto the heart wall before seed placement, and retracted from the heartwall after seed placement.

FIGS. 14A and 14B show a seed embodiment in which a seed pick-up coil1460 also serves the function as a distal attachment, extending into theepicardial space 1465. The seed includes a seed body 1402, the distallyextending coil 1460 and proximal tines 1465. The coil 1460 is wrappeddown in a delivery tube 1470 provided by a catheter 1412, and expands toits full diameter after being pushed into the epicardial space 1465. Theseed is pushed using a push rod, or wire, 1475 that operates to push thecoil 1460 from the distal opening in the delivery tube 1470 and into theepicardial space. The seed body 1402 and proximal tines remain withinthe heart wall 605. The expanded coil 1460 has the advantage ofcollecting more magnetic flux by virtue of its larger diameter, leadingto better coupling to the antenna, and a more efficient pacing system.The seed in FIGS. 14A-B can have a reduced diameter because it does notcontain a relatively bulky coil. The seed body 1402 contains thecapacitor and electronic components indicated in the schematic of FIG.4. Proximal tines 1465 are shown attached to the seed for additionalsecurement.

It is noted again, that it may be desirable to achieve maximum spacingbetween the proximal and distal electrodes to ensure conduction throughthe maximum volume of refractory tissue. For example, it may be possiblefor the bullet shaped seed of FIG. 4 to become encapsulated in fibrous,non-refractory tissue. In this case, the current density in tissuesurrounding the fibrous capsule may be too low to cause depolarization.A solution to this problem is to use the furthest extremities of theseed as electrodes. For example, tines 715, 720, 750 and 755 (see FIG.7) may be plated with a suitable conductive material to serve aselectrodes that extend into the epicardial space. Current passingbetween the distal tines and the proximal seed electrode would then passthrough refractory tissues. As a further precaution, the proximal tines750 and 755 could be plated with a conductive material and serve as anextension of proximal electrode 710. Current passing between distal andproximal tines would encounter refractory tissues with a high degree ofprobability. Similarly, the epicardial coil 1460 of FIG. 14 may containa central conducting coil surrounded by an electrical insulator, whichis in turn coated with a conductive electrode material.

For completeness, shown in FIG. 15 is an alternative seed coilembodiment in which three orthogonal coils are wound on a singlesubstrate. The substrate may be made from a permeable material. Currentsinduced in each of the three coils would be rectified, and passed to asingle capacitor. In this embodiment, the orientation of the seedrelative to the transmit antenna is immaterial. This is importantbecause there is no coupling between a coil having its axis parallel tothe plane of the antenna, and it may not always be possible to implant aseed with its axis perpendicular to the plane of the antenna. The seedof FIG. 15 collects magnetic flux in each of three orthogonaldirections, so that maximum flux is collected independent of theorientation of the incident magnetic field.

The electrical parameters in the seed circuit of FIG. 4, and thegeometry of the antenna 260 of FIG. 6 may be optimized by the use of acomputer model for the response of the seed to the magnetic fieldgenerated by the antenna. The fundamental requirement is that the energystored on capacitor 405 of FIG. 4 after charging is complete be equal tothe pacing threshold energy for the tissue surrounding the seed. Forexample, conventional pacemaker electrodes deliver on the order of fourmicro-Joules (E₀=4 μJ) of energy to pace the tissue each time the heartbeats. This number depends upon the tissue type, pulse shape, andelectrode geometry, but will be used here as an example. The totalenergy required to pace N sites is then on the order of N times thethreshold energy E₀). For example, if ten sites are paced using tenseeds, then the total energy requirement wilt be on the order of NE₀=40μJ for every heart beat. The energy that must be supplied by the antenna260 on each heartbeat is this minimum pacing energy times the overallefficiency of coupling energy from the antenna to seeds.

The energy delivered to each seed in a charging time, τ, may be computedfor a given set of seed circuit parameters and a measured or computedmagnetic field versus time at the site of the seed in question. This ispossible because the voltage induced in coil 410 is known to be equal tothe time rate of change of magnetic flux linking the coil. The stepsneeded to compute the energy stored on a given seed capacitor are:

For a given antenna shape, location and orientation, and antenna currentwaveform, I(t):

1) Compute the magnetic flux linking a seed coil 410 at a given locationand a given orientation relative to the antenna, residing in a tissuemedium having realistic frequency dependent values of electricalconductivity and permittivity.

2) Compute voltage induced in the coil (and modeled as a voltage inseries with the coil 410) as the time rate of change of the fluxcomputed in step 1).

3) With the switch 418 in position 1, use seed circuit equations tocompute the charge on capacitor 405 versus time, and therefore theenergy stored on the capacitor (equal to square of charge divided by twotimes the capacitance of 405).

Generally speaking, the magnetic field falls off rapidly as theseparation between the seed and the antenna increases. While this maynot be true for very large antennas, the body dimensions limit thepractical dimensions of the antenna. The exact location (and orientationif the seed does not have a tri-axial coil) of the seed will determinethe antenna current magnitude and ON-time required to charge that seed.The seed that links the least magnetic flux from the antenna will thendetermine these antenna parameters, since all seeds must be capable ofacquiring the threshold energy for pacing. We may refer to this seed asthe “weakest link”, and it alone will be used to compute optimal antennacurrent waveform and coupling efficiency.

The energy coupling efficiency is defined as the ratio of the totalenergy delivered to the seed capacitors, NE₀, divided by the sum of allenergy lost by the antenna during the on-time. Antenna losses that maybe included in simulations include:

Energy delivered to all seeds=NE₀

Power dissipated (as Ohmic heat) in seed circuit during charging

Power dissipated (as Ohmic heat) in antenna circuit during charging

Power dissipated (as Ohmic heat) by eddy currents induced in conductivebody tissues

The energy coupling efficiency is then given by NE₀ divided by the sumof losses listed above over the duration of the charging time. The Ohmicheat in the antenna circuit is primarily due to I²R losses in theantenna itself, and hysteresis losses in any magnetic materials that maybe included in the antenna design. This statement is also true for Ohmicheating in the seed circuit. Once the parameters of the antenna currentwaveform needed to charge the weakest link seed to the pacing thresholdenergy have been determined, these losses may be computed. Once theantenna current waveform parameters have been determined, the electricE, generated at any point in the body may be computed. Then, given aknowledge of the electrical conductivity of all body parts affected bythe antenna, the current density may be computed at any point in thebody as J=σE, where σ is the electrical conductivity at that point. TheOhmic heating due to eddy currents is then found by integrating thepower loss density J·E=σ|E|² over the volume of the patient's body.Since both the magnetic field and the electric field produced by theantenna waveform at any point in space may be derived from the magneticvector potential, the following further steps may be used to computecoupling efficiency:

4) Compute the vector potential, A, arising from a given currentwaveform in the seed medium, using realistic tissue conductivity andpermittivity.

5) Compute the magnetic field at the site of the seeds as B=curl(A)

6) From 5) determine antenna current waveform parameters needed tocharge the weakest link seed to the pacing threshold energy

7) Compute antenna circuit losses for the current waveform found in 6)

8) Compute the sum of all seed circuit losses given a set of seedlocations and orientations to the field, and the field computed in 5)using 6)

9) Compute the electric field at points in space as E=−∂A/∂t

10) Integrate σ|E|² over the patient's body using known or estimatedvalues for the electrical conductivity σ at each point in space todetermine energy lost to absorption by body tissues

11) Compute efficiency as charging energy delivered to seeds divided bythe charging energy plus the losses computed in 7)-10)

Optimization of seed design, antenna design, and antenna circuitwaveform is performed by iterating steps 1)-11) to maximize couplingefficiency. The lifetime of the transmitter battery is readily computedfrom the energy coupling efficiency since on each heart beat the antennamust supply the total pacing energy, NE₀ divided by the couplingefficiency. The total energy contained in the battery is its volumetimes its energy density. The total expected number of heartbeats thatthe system can pace is then the total battery energy times the energycoupling efficiency divided by the pacing energy per heartbeat, NE₀.Making an assumption about the average heart rate, say 72 beats perminute, then yields the battery lifetime in minutes.

In one example calculation a seed contained a coil 3 mm long by 2 mmdiameter wound on a core with relative permeability equal to ten. Thecapacitance was chosen to make the coil resonant at the frequency of theapplied magnetic field. A further constraint was made by choosing the Qof the coil (resonant frequency divided by the width of the resonancepeak) equal to ten. This constraint of a modest Q provides a margin forpossible frequency dispersion by conductive tissues, and a manufacturingmargin. Given these assumptions it was found that a magnetic fielddirected along the axis of the coil must have a magnitude of about 0.001Tesla (1 mT) to provide the minimum pacing energy of 4 μJ. The antennamodel in this calculation was a five inch diameter circular loop ofcopper having a total weight of 100 grams. The tissue model employed wasa combination of heart muscle and blood, having about the sameelectrical conductivity. When the weakest link seed was placed at adistance of three inches from the plane of the antenna, the followingwas determined: The optimal energy coupling occurred at a frequency ofabout 30,000 Hz (30 kHz), where efficiency peaked at about 0.5%, and thelifetime of a 100 gram battery with 720 Joules/gram energy density wasabout 2 months.

The efficiency can be improved by improving magnetic coupling betweenthe seeds and the antenna. This may be accomplished by using multipleantennas, for example one loop on the ribs over the anterior side of theheart, and one loop on the ribs over the posterior side of the heart.Two or more antenna loops may insure that the weakest link seed iscloser to a loop than the three inches used in the example above. Analternative location for an antenna loop may be a loop inserted into theright ventricle of the heart, and attached to a controller placed at theusual pectoral implant location. Such a loop would be located closer toall seeds, particularly since the antenna is energized during systolewhen the heart is contracted.

Battery lifetime can be extended indefinitely by employing arechargeable battery. The battery may receive energy for recharging byinductive coupling to antenna 260. External antennae and transmittersfor recharging could be located under or around the patient's bed orchair, or be integrated into special clothing. As an alternative to arechargeable battery, the antenna, transmitter, and battery of FIG. 3could be integrated into clothing or a disposable patch worn by thepatient. ECG signals needed to time the seed pacing could be receivedvia an inductive link from a conventional pacemaker with right atrialand right ventricle leads. In this case, elaborate antenna designs couldbe incorporated into the special clothing. For example, the antennacould have a portion that surrounds the chest at the latitude of theheart.

FIG. 16 shows a schematic diagram of an antenna 260 with the chargingcurrent waveform being supplied by capacitive discharge through theantenna 260, and capacitor recharge provided by a battery 1605. Thevalue chosen for the capacitor 1610 determines if the current waveformhas a single peak or whether the current rings down in a damped sinewaveform. Communications electronics 1615 sends pacing discharge signalsto the seeds, but may also receive ECG signals from the seeds or aconventional pacemaker. The charge electronics 1620 receives energy viathe antenna from an inductive link to an external antenna, to rechargethe battery. A control circuit 1625 controls the operation of therecharge circuit 1620 and the communications electronics 1615.

It is also noted that alternative sources of power for the seeds may beused. For example, the mechanical energy of the beating heart is manyorders of magnitude larger than the energy required to pace the seeds.At the site of a seed, the heart muscle thickens during systole andthins during diastole as the heart beats. It is estimated that a one mmdiameter transducer placed across the heart muscle could generate 65 μJof energy due to the contraction of the heart, more than ten times theenergy needed to pace. A simple mechanical to electrical transducerhaving nominal efficiency could provide the energy to pace a seed. Otherminiature local sources of energy have been suggested in recentliterature. These include: piezoelectric and electro-active polymermaterials that transduce mechanical to electrical energy; bio-batteriesthat convert body heat and/or blood flow energy to electrical energy;and tiny amounts of radioactive material that emit short range alpha orbeta particles that are readily shielded.

In addition, the seed circuit of FIG. 4 can be simplified by omission ofthe capacitor and voltage controlled switch. That is, the seed circuitmay consist simply of a coil connected across electrodes in contact withtissue. In this case a magnetic field pulse induces a voltage pulse inthe seed coil, and the induced voltage directly discharges into tissue.If all seeds are the same, pacing of all seeds is simultaneous. However,the rise time of the induced voltage can be adjusted by adjustment ofthe coil parameter number of turns, core permeability, and adjustment ofa resistor in series with the coil. Thus, a collection of seeds havingvarying rise times may be used to synchronize the firing sequence of theseeds. The controller may sense a singe local ECG, for example theatrial or right ventricle electrode of a special transmitting seed or ofa conventional pacemaker that transmits data to the controller. A burstof current into the antenna would then fire all seeds, with the precisetime of firing determined by the electrical properties of each implantedseed.

FIGS. 18A-18C show an end view, side view, and side view with equivalentcircuit for a simplified seed 1800 for delivering stimulation to tissue,including myocardial tissue on the inside of a heart chamber. As shown,the seed does not have separate energy storage components such as abattery or a capacitor. It instead is comprised of a ferrite core 1805which may be in the form of a cylinder approximately one mm in diameterand three mm tong. At each end of the core 1805 are ferrite caps 1810which may be in the form of circular disks about 1 mm thick and about 3mm in diameter. The caps 1810 may be attached to the end of the core1805, may have central holes through which the core 1805 is received, ormay be integrally formed with the core 1805. Ring electrodes 1815 may beformed about the periphery of each cap. The ring electrodes 1815 may beformed of any appropriate materials such as platinum-iridium alloy. Thering electrodes 1815 may be bonded to the caps 1810 using medical gradeepoxy, cyanoacrelate, or the like. Other arrangements for the electrodesand other components may also be used, and the particular layout andshape of components that is meant to be illustrative rather thanlimiting. Because the seed does not have a distinct energy storagedevice such as a battery or capacitor, it is referred to in thisdocument as a direct activation electrode assembly or device.

The seed 1800 may receive signals using a long loop of wire 1820 wrappedaround the core. For example, 99.99% silver wire that is 0.002 inches indiameter and is covered in a polyurethane nylon insulation may be used.The wire 1820 may be wrapped around the core 1805 in any appropriatemanner and may comprise, for example, about 900 turns of wire. Ingeneral, the voltage induced in the coil is proportional to the numberof turns of wire. Wire having a smaller diameter yields more turns whenthe wire fills the empty volume over the core (nominally 3 mm long gapwith 3 mm outside diameter and 1 mm inside diameter). However, smallerdiameter wire has a higher electrical resistance, and if the coilresistance becomes comparable to the impedance of the tissue beingpaced, the net energy delivered to the tissue will diminish. In generalthe electrical resistance of the wire should not exceed a few hundredOhms. The measured electrical resistance of the 900 turns of wire 1820is about 60 Ohms.

The seed 1800 may also be covered as appropriate to protect thematerials in the seed 1800 and to insulate them from the tissue andfluids around the seed 1800. For example, a hermetic epoxy layer 1830may be applied to the ends of both caps 1810, and another hermetic epoxylayer 1825 may be applied around the outside of the coiled wire 1820. Ingeneral, the ring electrodes will not be insulated, though they mayotherwise be treated, so that they can deliver sufficient energy to thetissue surrounding the seed 1800. The coil 1105E and/or one or more oftines 1110E, and/or the seed distal curved face 1123E may beelectrically connected to and part of the distal electrode 1135E.Alternatively, one or more of 1105E, 1110E and 1123E may be used inplace of ring 1135E as the distal electrode.

In general, the seed 1800 should be small enough to be delivered easily,such as through a 9 French delivery catheter. Exemplary dimensions ofsuch a seed are 5 mm long and 3 mm in diameter. Also, the seed justdescribed may be incorporated with the delivery and anchoring mechanismsdiscussed earlier in this document. Typical parameters for the seed 1800would be a voltage pulse amplitude greater than 0.5 volts (with 2 voltsbeing typical), and a pulse duration of approximately 0.4 msec. Inaddition, to neutralize charge on the electrodes, the electricalwaveform that seed 1800 delivers to the tissue will generally have thepacing pulse described above (with the distal electrode being thecathode) followed by a smaller-amplitude, longer-duration pulse of theopposite polarity so that the integral of the waveform over time will bezero.

Advantageously, the described seed is extremely uncomplicated and isthus capable of delivery one or more specific benefits. First, thesimple design allows the seed to take a very small form factor. A smallseed can be used with less tissue trauma to a patient, and may also beimplanted more easily and at more locations using, for example,percutaneous transluminal implantation with catheters, as discussedabove. This form factor can be reached without extreme engineering forminiaturization, such as would be required for a system using electricalstorage devices in the seed.

The simple design is also likely to provide excellent reliability, asthere are very few parts to the system, and very little to wear out orotherwise fail. The simple design also contributes to manufacturability,as the seed is fairly simple to make, and thus should be lower in costand also be manufactured with fewer errors. In addition, the describedantenna circuit is small and simple, which may facilitate implantation,lower costs, and improve manufacturability and reliability in similarways.

The simple seeds also provide operational flexibility. Specifically, thepacing waveform parameters may be adjusted at the antenna circuitwithout a need to communicate with each of the multiple implantedwireless electrodes. In addition, the seed can provide extremely fastrise times (e.g., an “instant ON” characteristic), which allows possiblevoltage limiters in the seeds to give all electrodes the same pacingpulse amplitude with nearly the same rise time.

The equivalent circuit attached to seed 1800 in FIG. 18C is designed torepresent the features of tissue around the seed 1800. The equivalentcircuit comprises two parallel impedances 1830, 1835, with impedance1830 representing extra-cellular conductive fluid with a resistor, andimpedance 1835 representing muscle cell impedance by cell capacitance inseries with a resistor representing intra-cellular fluid. The equivalentcircuit is useful in testing candidate wireless electrode or seeddesigns to determine which will provide the best treatment underparticular conditions. The equivalent circuit can also be used after thedesign phase, during manufacture, to test seeds to ensure that they areworking properly. For example, manufactured seeds can be placed in amagnetic field having a waveform substantially identical to that used inthe implanted systems, and their reaction may be measured to ensure thatthey meet manufacturing requirements. In this manner, the equivalentcircuit may be particularly useful in two phases of the process-designand manufacture.

The design of the seed can be expressed mathematically by starting withan expression for the voltage induced around the perimeter of an areaelement whose surface is perpendicular to a time varying magnetic field:V _(ind) =−A(dB/dT)  (1)where

V_(ind)=induced voltage in volts

A=surface area in m²

B=applied magnetic field in Tesla

In Eq. (1), the magnetic field is assumed to be constant in space overthe area of the surface. The induced voltage is present throughout spacesurrounding the source of the magnetic field. A current will flow in aconductive element placed in the time varying magnetic field. Forexample, the source of the magnetic field may be a current pulse flowingin an antenna, as described above. In a coil aligned with the externalmagnetic field, the voltage of Eq. (1) is induced in each turn of thecoil. If the coil is wound on a magnetically permeable core material,the voltage is further multiplied by the effective permeability of thecore. If the coil has multiple layers, the area of Eq. (1) is larger foreach successive layer.

Under these observations, the net voltage induced in a coil wound on apermeable core is:V _(ind)=−β(dB/dt)  (2)where

β=μN(π/12)(D_(i) ²+D_(i)D_(o)+D_(o) ²)

μ=effective permeability of the core (unitless)

N=the total number of windings on the coil

D_(i)=inside diameter of the coil in meters

D_(o)=outside diameter of the coil in meters

If the magnetic field is created by a pulse of current in the antenna,then the time integral of the induced voltage in Eq. (2) is zero,because the field itself is zero at both time zero and after the pulseis delivered. Such a seed thus meets the standard, discussed above, thatthe integral of the waveform over time is zero.

Considering now the case of a magnetic field generated by a circularloop antenna, the magnetic field at a distance, z, along the axis fromthe center of a circular loop carrying a current, I, is:B=(μ_(o) N _(a) I/D)[1+(2z/D)²]^(−3/2) =γI  (3)where

μ_(o)=permeability of free space=4π×10−7 Weber/Amp-m

N_(a)=number of windings on the antenna

D=antenna diameter is meters

z=distance along axis from antenna center in meters

γ=(μ_(o)N_(a)/D)[1+(2z/D)²]^(−3/2) in Tesla/amp

The current, I, through the antenna may be made a pulse whose timederivative yields an appropriate pacing waveform when Eq. (3) isinserted into Eq. (2). A relatively simple circuit, like that shown inFIG. 16 can produce an appropriate pulse. In that figure, the capacitor1610 may be charged to the voltage, V, of the battery 1605. Amicroprocessor controller, such as control circuit 1625 may beconfigured to operate the switch near capacitor 1610 and may sense thep-wave in a patient's cardiac ECG. The ECG may be sensed, for example,near the site of the controller implant, or via skin patch electrodes inthe case of an external antenna. Alternatively, an implanted sensinglead or wireless electrode may transmit the ECG signal or p-wave triggerto the controller. When the capacitor is switched across the circularloop antenna in FIG. 16, the current flowing in the antenna is given by:I=(CVQ ² /τS)[e ^(−(1+S)t/(2τ)) −e ^(−(1−S)t/(2τ))]  (4)

where

C=capacitance in farads

V=voltage applied in volts

Q=quality factor (unitless)=(1/R)(L/C)^(1/2)

τ=L/R (time constant) in seconds

L=antenna inductance in Henries

R=antenna and capacitor resistance in Ohms

S=(1−[2Q]²)^(1/2)

Combining Eqs. (2)-(4) provides the voltage induced in the wirelesselectrode coil:V _(ind)=βγ(CVQ ²/2τ² S)[(1+S)e ^(−(1+S)t/(2τ))−(1−S)e−^((1−S)t/2τ))]  (5)

By evaluating Eq. 5 numerically, one can determine that the waveform isa damped sinusoid when Q>0.5, and is a pulse waveform when Q<0.5. Apulse waveform is appropriate for pacing, and by numerical evaluation ofEq. (5), the pulse has maximum amplitude when Q=0.5. Thus, for thisidealized model, antenna components may be selected to achieve Q=0.5, sothat Eqs. (4) and (5) become (in the limit of Q→0.5 and S→0):I=(CVt/4τ²)e ^(−t/2τ)  (6)V _(ind)=βγ(CV/4τ²)(1−t/2τ)e ^(−t/2τ)  (7)

The waveform of Eq. (7) has a positive pulse with a zero crossing att=2t, followed by a shallow negative wave that falls exponentially withtime. The wave form of Eq. (7) integrates to zero, as is discussed aboveas being desirable. For a desired pulse width of 0.4 msec, τ is selectedas 0.2 msec. Equation (7) is shown plotted in FIG. 19, with a voltage attime zero taken as 0.23 volts. The solid line in the figure representscomputed values, while the triangles represent measure values using aseed like that shown in FIGS. 18A and 18B. Specifically, the measureddata was taken with a seed electrode body 5 mm long comprising a coilwound on a ferrite bobbin having core dimension of 1 mm and end flangethickness of 1 mm on each end—the coil of wire being 3 mm long with aninside diameter of 1 mm and an outside diameter of 3 mm, wound on theferrite bobbin with 900 turns of 0.002 inch insulated silver wire. UsingEq. (2), these parameters produce a value of β=0.003 m². Themeasurements were generated using an antenna having a diameter of seveninches that was constructed from four turns of AWG #8 copper wire.

The wireless electrode was placed at the center of the circular antenna,where the parameters of Eq. (3) yield γ=2.8×10⁻⁵ Tesla/amp. The antennacircuit capacitor had C=0.02 Farads, and the applied voltage was V=15volts. With τ=0.2 msec, the voltage at time zero computed from Eq. (7)and these parameter values is V_(ind)=0.16 volts, compared toV_(ind)=0.23 volts in the computed plot of FIG. 19.

Further testing was conducted on seeds having end caps of varyingthickness, with the coil wound on a 1 mm ferrite core and the gap filledwith wound insulated silver wire. The seed with the highest inducedvoltage had end caps 1 mm thick, with 3 mm of wound wire between them,and a total diameter of 3 mm.

This seed was tested with and without the equivalent circuit of FIG. 18Cattached to the electrodes. FIG. 20 shows a plot of the voltage inducedin such a seed when it is placed at the center of the seven inchcircular loop antenna discussed above, with voltage V=15 volts andconductance C=0.02 Farads. The figure indicates that the wirelesselectrodes are not loaded down significantly by the tissue impedance,and pacing voltages larger than one volt are readily attained in thepresence of tissue. The waveform of the figure is also appropriate forcardiac pacing using a simple and small wireless electrode and simpleantenna circuit. A comparison of FIG. 20 without the equivalent circuitand FIG. 19 shows that the seed has an effective permeability of1.8/0.18=10 (equal to the ratio of peak induced voltages, since theseeds have the same geometry and number of turns).

A passive voltage limiting element such as a Zener diode may be added tothe seed across the stimulation electrodes to control the voltage pulseamplitude. For example, when multiple seeds are located at multipledistances from the antenna, the magnitude of the applied magnetic fieldwill vary from seed to seed according to Eq. (3). The voltage limitingelement may help ensure that the pulse amplitude is the same for allseeds and all antenna configurations when the seeds are close enough tothe antenna to generate the limit voltage.

A number of embodiments of the invention have been described.Nevertheless, it will be understood that various modifications may bemade without departing from the scope of the invention. For example,although the disclosure discusses embodiments in relation to cardiactissue, the systems and methods described herein are applicable toexcitation of other cells, tissues, and organs that may be stimulated toachieve some benefit or result.

In some embodiments, the systems and methods described herein may beused in certain neurological applications. For example, the wirelesselectrode assemblies and the related systems described herein may beemployed to limit pain, control muscle spasms, prevent seizures, treatneurohormonal disorders, and the like.

In other embodiments, the leadless electrode assemblies may be deliveredthrough other conduits other than blood vessels. For example, wirelesselectrode assemblies described herein may be delivered through theesophagus to the stomach lining or other tissue in the digestive tract.By using the electrode assemblies to electrically stimulation of thestomach tissue or other tissue in the digestive tract, the systemsdescribed herein may be used to treat digestive disorders or controlhunger sensations.

In certain embodiments, the wireless electrode assemblies describedherein may be deployed in the urogenital tract. In such embodiments,organs tissue in the abdominal area may be accessed percutaneously viacatheters through the peritoneal space.

Also, the apparatuses, systems, and methods described herein and relatedto leadless stimulation of tissue may be combined with elements of othertypes of seeds and/or related apparatuses, systems, and methods. Suchelements may be other than those described in this document, such as theseeds, also referred to as microstimulators, and related elements ofapparatuses, systems, and methods described in co-pending applicationSer. Nos. 10/607,963; 10/609,449; 11/034,190; 11/043,642; 10/607,962;11/043,404; 10/609,452; 10/609,457; and 10/691,201, each of which isassigned to Advanced Bionics Corporation, and each of which isincorporated herein by reference in its entirety.

For example, the microstimulators described in these applications may beemployed as seeds (modified so as to provide an appropriate excitationor stimulation signal), may be provided with the delivery and attachmentor anchoring features described herein, and may be implanted using thedevices and methods described herein. Alternatively, the apparatuses,systems, and methods related to seeds as described herein may bemodified so as to include at least one element of the apparatuses,systems, and methods related to microstimulators described in theseincorporated applications. Such at least one element may relate toimplantation and/or explanation; fixation and/or anchoring or seedsand/or microstimulators; power transfer and/or data communicationbetween seeds, microstimulators, and other implanted or external powertransfer and/or data communications devices; methods of manufacture;electronic circuitry; mechanical packaging of hermetically-sealed seedsand/or microstimulators; materials; and all other elements ofapparatuses, systems, and methods described in these incorporatedapplications.

What is claimed:
 1. A leadless pacing device comprising: a main bodyincluding a battery and first and second electrodes configured todeliver electrical stimulation to a heart, the first electrodepositioned at a distal tip of the main body; and a plurality of tinescoupled to the main body that are configured to engage the heart, theplurality of tines configured to self-bias from a first configurationfor delivery to a heart chamber into a second configuration for engaginginto a wall of the heart chamber, wherein the tines extend distal of thefirst electrode when moving between the first configuration and thesecond configuration, and wherein each of the plurality of tinesincludes a portion extending distal of the first electrode when in thesecond configuration and a distal tip that extends proximal of the firstelectrode when in the second configuration.
 2. The leadless pacingdevice of claim 1, wherein the second electrode is located near aproximal end of the main body.
 3. The leadless pacing device of claim 2,wherein the second electrode is a ring electrode.
 4. The leadless pacingdevice of claim 2, wherein the first electrode is configured as acathode and the second electrode is configured as an anode, wherein thefirst electrode is configured for engagement with the heart wall fordelivery of the electrical stimulation when the second electrode isexposed to blood in the heart chamber.
 5. The leadless pacing device ofclaim 1, wherein the plurality of tines extend from a distal end of themain body in the second configuration.
 6. The leadless pacing device ofclaim 5, wherein the plurality of tines extend from a location at aperiphery of the distal end of the main body in the secondconfiguration.
 7. The leadless pacing device of claim 1, wherein theplurality of tines are configured to extend in a longitudinal directionnear the periphery of the main body in the first configuration duringdelivery to the heart chamber.
 8. The leadless pacing device of claim 1,wherein the main body is cylindrically shaped.
 9. The leadless pacingdevice of claim 1, wherein the second configuration is a curledconfiguration.
 10. The leadless pacing device of claim 1, wherein thesecond configuration is a hook configuration.
 11. A system, comprising:a leadless pacing seed including a main body including a battery and twoelectrodes configured to deliver electrical stimulation to the heart,the leadless pacing seed including a plurality of tines coupled to themain body that are configured to engage the heart; and a deliverycatheter, wherein the leadless pacing seed is configured to bepositioned within the delivery catheter during delivery to the heart,wherein the plurality of tines include a longitudinal configurationduring delivery and a hooked or curled configuration when implanted,wherein a distal tip of each of the plurality of tines points in adistal direction when in the longitudinal configuration within thedelivery catheter, and wherein the distal tip of each of the pluralityof tines points in a proximal direction when in the hooked or curledconfiguration when deployed from the delivery catheter, and wherein thedistal tip of each of the plurality of tines extends proximal of atleast one of the two electrodes when in the hooked or curledconfiguration.
 12. The system of claim 11, wherein, during delivery, thedistal tip of each of the plurality of tines is constrained into thelongitudinal configuration and is bent towards the longitudinal axis ofthe delivery catheter.
 13. The system of claim 12, wherein the pluralityof tines are configured to spring outwardly into the hooked or curledconfiguration when the leadless pacing seed is pushed out of the distalend of the delivery catheter.
 14. The system of claim 11, furthercomprising a tether configured to couple the leadless pacing seed to thedelivery catheter during delivery.
 15. The system of claim 11, furthercomprising an extraction pull wire configured to pull the leadlesspacing seed proximally within the delivery catheter.
 16. A leadlesspacing device comprising: a main body, the main body including a batteryand first and second electrodes configured to deliver an electricalpulse to a heart wall, the first electrode positioned at a distal end ofthe main body and the second electrode spaced proximally away from thefirst electrode; and a plurality of tines configured to secure the mainbody to the heart wall, the plurality of tines biased to extend from adistal end of the main body in a curled or hooked configuration, whereinthe leadless pacing device is configured to attach to the heart so thata threshold pull force of greater than 0.3 pounds is required to removethe leadless pacing device from the heart wall, wherein each of theplurality of tines includes a distal tip that extends proximal of thefirst electrode when in the curled or hooked configuration; and whereineach of the plurality of tines includes a curved portion that extendsdistal of the first electrode when in the curled or hookedconfiguration.
 17. The leadless pacing device of claim 16, wherein thethreshold pull force is greater than 0.5 pounds.
 18. The leadless pacingdevice of claim 16, wherein the threshold pull force is greater than 1.0pounds.
 19. The leadless pacing device of claim 16, wherein the mainbody is cylindrical, sized and shaped for intravascular delivery to alocation within the heart, and sized and shaped to fit entirely withinthe heart.